I agree with Secespitus' comment.
You should always strive to ensure that edits don't invalidate existing answers.
Sometimes, that means to swallow your pride and accept that the question turned out differently than you intended. I've had that happen to myself on more than one occasion, not just here on Worldbuilding.
Rather than making an edit that invalidates existing answers (even more so when you know it does rather than just that it might), you should leave the question as it is, maybe even add a point such as (in this case) "if you feel FTL drive makes this easier, then you may use that in your answers". Then, post a new question, which posits from the beginning a universe in which FTL drive does not exist. Link to your previous question. Point out the difference. Highlight it, put it at the top, as a heading, whatever it takes, but make sure that people know about the difference of the two seemingly very similar questions up front. By having such a key difference, the two automatically are non-duplicates, because answers valid to one are not valid to the other.