I need to replace a pair of 26er mtb wheels and want to see if my qr fork and frame can be adapted for through axle hubs.
YES. You will need a adaptors called "end caps" that fit the ends of the axle (front&rear) and make it compatible with the use of QR. Depending on the brand of the hubs, the "end caps" can be found on the market - otherwise will need to make it at a turnery (it's a simple work). But there are exceptions! Some front hubs of type "Boost" are 15mm but slightly wider (110mm vs. 100mm) than traditional QR and 15mm hubs. If the wheels you buy come with this hub, it will not works even using "end caps". Before buying future wheels, make sure there are available in the market "end caps" for them.
Yes, as the above answer has explained a some 15mm TA front hubs can be converted to use a 9mm QR.
No, a rear 142x12 TA hub cannot be coverted to fit in a 135x10 frame as there is a 7mm width difference.
Also you ask if your frame or fork can be converted and the answer to that is also no. The open ended dropouts do not have a simple (and safe) solution for through axle conversion.