(How) Can I match new 3/4" T&G oak flooring with old 7/8" flooring?
If you cut all the tongues off then the floor will likely squeak... And good luck with that...
Get a shop to make the replacement flooring or buy 7/8 timber and a router and sort it yourself. If you think they will charge too much (after doing it yourself you might think they made a good offer :) )