I'm not sure if I understand the situation correctly, but the testosterone is exactly the same for subcutaneous and intramuscular. Sometimes the vials will say "for intramuscular use only" on them, but I've always been on subcutaneous and it's fine. If you've been doing sub and have the needles for that, just continue with that method.
No you can't use the same needles; the needles will be too short to reach the muscle. But you don't need to worry about it, it's the same hormone either way.