Don't scratch the fretboard. If you're not planning on using the frets you rip out, then you can do whatever to get them out.
1)make sure the varnish doesn't bubble
2)if you're using a chisel to force frets up, use a larger chisel, sounds dumb, but it spreads the force of blows, so won't dent the fretboard.
3)when you fill in the frets, make sure it is completely flat
4)don't just do an ok job, put lots of time and effort into it, and do the best you can