I want to change my sub to inverted, can i put the screws back in the old holes or is that a bad idea and will become loose.
See what they feel like when you tighten them. If they tighten up nice you should be fine but if they are stripped......
If when you installed the speaker you drilled the correct piolet hole and made a good cutout so that the screws are not right on the edge all should fine. I have changed drivers in the same box many times and never had one come loose yet.
If you have trouble fill the old holes and rotate the speaker a bit and start over.