Found reply for a research application from 2 months ago in my junk mail
Suppose you do nothing. Then you remain at your current position, yourwork carries on, and whatever may come of that will happen.

Suppose you respond to this professor's request. You may write something that apologises for the late response, asks whether the position is still available, and provides your most recent transcript.

Then at worst, they reply with rudeness and aggression, and you thank them for their time, then move on. They may tell you the opportunity is closed, but invite you to keep in touch or to apply again when you graduate/advance in qualification.

They may also say the opportunity remains. At that point you must decide whether it is better to


thank the professor and remain where you are (flakey, might be seen as rude),
open a dialogue that allows you to transition once current projects are complete (reasonable, but may not work for the professor or their lab),
try to develop a sideline partnership working in or with both labs (could be a huge job, but also huge rewards if it works), or
jump ship totally (also flakey, reflects poorly on your commitment).