So I've finally gotten Truro to League 2, and the board granted my request to go professional. Problem is, I offered all my players new contracts when we were still semi-pro, so they're all on part-time contracts. The board said that everyone would move over to full-time contracts in the near future, but does that happen automatically or will I have to wait to offer all my players new fulltime contracts?
No one will move to FT contracts automatically. They have to be offered new contracts. As far as I know, there is nothing stating that you can't leave them on PT contracts until they run out, though the players might be asking for new contracts before then.
Just be aware that they wage expectations skyrocket once you become Professional. I have seen players accepting a £300/pw full-time contract while the club was Semi-Pro, and then suddenly want £900/pw as soon as it turns Professional. And sadly the wage budget normally doesn't triple to meet these new expectations.
I usually have to keep only the best on FT, until the wage budget rises, and the youths/reserves on PT (or Yth if possible).
Although it's true your current players want a large wage increase to go from semi pro to pro, it doesn't seem to affect new signings, who want a similar amount regardless of professional status in my experience. You should be able to replace your current squad pretty easily with better players on full-time deals for similar money.
Yep, keep to short term contracts in the lower league so that when you inevitably go professional, you can get rid of the deadwood and replace at a more realstic price. Just keep the spine of the team.