Can I still use my ACU backpack while wearing the new OCP ACUs until i can afford the OCP Bag?

Yes. OCP TA-50 is still being phased in (ie IOTV, rucksacks, cold weather gear, etc). Until it is fully phased in, there is allowance for mix and match. What they don't want you doing is mixing and matching ACU parts.

The SMA said it was ok to mix and match the OCP uniform and parts (follow the rules put out). If you are in ACUs though you cannot mix and match - it usually must all be ACU foliage pattern. It will take a while to phase out all the Field Issue items from the ACU foliage pattern to the OCP woodland pattern.

I personally use a Black Backpack since it is a optional part of the regs, that way I don't have to worry about adapting to the new uniforms if they decide to change it again.