Originally Posted by
jon c.
You may need to change deraillers, but I think you'd be happiest moving to a triple in the front. 26-36-48 would seem like it would suit you well.
+1
Going from a 42t to a 39t will help marginally, but not all that much. It's roughly comparable to a change by 2 teeth in the back. Going o a larger freewheel (you didn't mention the size) would also help, possibly more depending on what you have there now. However there's a limit to what you can do by changing a chainring or freewheel alone.
Trying to broaden the range of gearing with your 7s will mean reducing your mid range gear selection. Going to a triple will allow you to maintain decently close gearing in the mid range where you do most of the riding, yet allow for an ultra low set of gears for climbing walls.
This is a more expensive option, but from your OP, I suspect something you'll end up doing anyway, so you're better off biting the bullet now rather than spending dough for a 39t chainring that won't solve the problem.