I’m a fully vested founder, what challenges will I face raising our seed round?
None. If you are upfront about it, and then when asked, acknowledge you are OK adding (back) vesting.
To simplify David S. Rose‘s answer — just tell the new investors.
If they are true angels, e.g., small checks at a Demo Day, they may not care.
All VCs will care. They’ll usually ask you to re-vest all or some. As long as you are upfront you are currently 100% vested, but know it needs to change, it won’t be an issue.
Sometimes, if it’s a small round, a small VC may “punt” and let the Bigger VC in the next round deal with the issue. That may be OK but again only if you are transparent up-front.
Also, bear in mind — this is a good chance to fix a situation. The more committed founder strongly benefits when a less committed founder leaves with less vesting. If you are all-in, and control the company, and ain’t going anywhere … you may well benefit a lot from re-vesting all the founders.