Yes, it is possible and you can ride with the smaller-sized 27.5 wheels on a larger-sized frame and fork. but you have to go through several phases to do this setup because the crankset would lower down and bike pedals would also sit down due to the smaller-sized wheels installed on a larger-sized frame. the pedal height lowers its position by 3 to 4 inches and the larger-sized 29 wheels stay 3 to 4 inches higher than the smaller wheels. although it is normal you have to avoid hard and aggressive terrains with the same setup, due to the lower height difference, and this setup is usually recommended for normal and smooth terrains without drops and up and downs.
And the other major difference is that smaller-sized wheels provide a better grip for handling on the terrains compared to larger-sized wheels. larger wheels have greater rolling power and quick response as compared to smaller wheels, due to which these smaller-sized wheels have greater rolling resistance as compared to larger 29-sized wheels. increased rolling resistance would decrease bike speed but gives excellent bike gripping and handling on the trails.
Boost And Unboost Difference:
There are two types of hub axles boost and un-boost, boost axles have larger with the greater diameter and un-boost axles have a smaller width and narrower, based on this difference you have to make 27.5 wheels compatible with the 29-sized frames, if the previous 29 wheels had boost hubs and new inserting smaller sized 27.5 wheels have un-boost hubs, then the setup won’t work because boost and un-boost brings a noticeable difference and are non-compatible with each other.
Fork Difference:
27.5 forks are recommended and are compatible with the 27.5 wheels also you can insert large 29-sized wheels on these 27.5 forks because the setup is compatible but if your 29 frames have 29 forks and you are swapping and interchanging 29 wheelsets with 27.5 wheels, this setup will not work and is compatible with each other, because the thickness and width of the 29 forks and 27.5 forks have a noticeable difference and are non-compatible. the diameter and thickness of the forks start from 2.5 to 3.0, while inserting a different sized fork on different sized wheels, you have to choose the right diameter of the fork which should match exactly with the bike frame and smaller or larger wheels.
If you want to replace your fork that should be compatible with both 29 and 27.5 sizes then you can buy 29-sized boost forks, theses boosted forks are compatible with both smaller and larger-sized wheelsets. but if you want to buy a 27.5 fork so that it would be compatible for both 27.5 and 29 wheelsets, then never buy simple 27.5 forks, buy 27.5 plus forks, its a sign of the forks that these forks are compatible for both smaller and larger sized wheelsets.
Boosted forks are compatible with wider tires and you have both options, you can install narrower tires according to your own choice on boosted forks as compared to unboosted forks.
Height Of The Fork Increases:
29 fork sizes are higher and bigger almost an inch or two from the lower sized 27.5 forks while mounting smaller sized wheels with larger fork and frame would higher the bike and you feel like you are gripping your handlebar at a high level as compared to the normal bikes.
And also when you sitting on the saddle of your bike while riding on the trails, then smaller-sized wheels would not push your body in a forwarding direction but on larger wheels, it is common, when you pull the levers of the front and rear brakes, your body would slide in the forward direction, but it is not common with 27.5 wheels on 29 forks and frame.
More Grip On Twisting And Corner Trails:
Smaller-sized wheels with a larger fork and frame would provide greater grip on corners and twisted terrains as compared to larger-sized wheels, it is the major advantage of 27.5 wheels with a larger fork and frame, most riders will do this setup for better gripping and handling.
Crank Legs:
29 frames have larger cranksets for larger sized 29 wheelsets, but for 27.5 wheels you have to replace larger crank legs with smaller crank legs so that these should be compatible with the smaller sized 27.5 wheels.
Bottom Bracket BB:
The size of the bottom bracket should match exactly with the smaller wheelsets without any issue, but if it has a larger size and it does not match exactly with the 27.5 wheelsets then you have to change it with a smaller BB to make the setup compatible and running smoothly.
The main thing is the height of the BB from the ground surface 29-sized BBs have larger sizes than 27.5 brackets and surely there would be a noticeable difference in their heights from the surface ground.
Chainstay Lengths:
The length of the chain and chain stay tube is different for 29 and 27.5-sized wheels and frames, make sure chain stay lengths are staying accurately without any default or issue.
Head Angle:
It is the steerer tube angle that makes a lot of difference for smaller and greater sized wheelsets, you have to maintain and adjust accurately the head tube angle same as it was with the larger 29-sized wheels.
Seat Tube Angle:
It is the angle of the lower seat tube that is reaching down towards the rear wheel. larger wheelsets have different seat tube angles because 29 wheelsets are compatible with 29 size frames and forks and these are not recommended for smaller sized wheels and 27.5 wheelsets have their own recommended and compatible 27.5 size frames, but this setup in which you are going to swap larger wheels with the smaller one on a larger frame would change everything related to the frame hub axles and forks.
Increased Pedal Strikes:
Smaller-sized wheels also increase the number of pedaling strikes on the trails as compared to larger-sized wheels. this difference is just because of the size of the wheelsets larger wheelsets take less time in completing one complete rotation as compared to smaller wheelsets.
Rim And Tire Setup On New Frame:
New smaller sized 27.5 wheels on a larger frame and fork would require proper maintenance, you have to inspect the tracking of the rim and tire within the wheel and maintain the distance of the tire and rim from brakes to avoid rubbing and scratching and also check the tightening of the wheel to avoid wobbling on the terrains.
The width and thickness of tires would also matter for swapping from larger to smaller sized wheels, you have to check the tires for replacing or interchanging wider tires with narrower and narrower wider tires.