Hey Blake,
This is unfortunately the least ideal scenario because falls into a capability gap between the two official conversion utilities. The Account Conversion Tool doesn't cover Forms at all, and the Trustee Migration Utility covers Forms but can only convert repository users to LFDS Laserfiche users. We're aware of the gap and are working on a new utility that should hopefully cover all the cases with full product coverage.
LFC made a custom build of the Account Conversion Tool that added partial Forms support (completed with some emergency custom SQL scripts) to handle this specific case for one customer. It was at least twenty hours of effort to successfully deploy, and it has not been tested or used outside of this one case. As there's not really a better alternative at the moment, I can provide it to you with the clear understanding that it's absolutely "as-is" and Laserfiche Support won't be able to provide any assistance in running or troubleshooting it.
If you're interested, please reach out to me directly. Otherwise, I'd either see if it's remotely possible to have users finish out their tasks, add all the Windows accounts, and re-configure Forms access rights. I know that sounds tedious, but I promise that it's better than digging through the Forms database trying to figure out which tables user SIDs didn't get updated in. You could also wait to see if the upcoming new user conversion utility can address this use case (first beta build targeting LFDS Laserfiche to SAML user conversion was recently completed).