Either way works for replacing caps. I've done both ways, and it just depends on how microscopic I want to get. The perceived results are always better, the more effort involved :-) The actual results, assuming tight connections, of course, are pretty much equivalent.
I don't see any harm in opening up some space around the crossover. I'd be worried, actually, on finding melted *anything* inside a speaker. It's not that common, but it does happen, often capacitor-related.