This is a fresh C# port (Net.NowhereAtAll.Xfty) — there is no prior release
of it to migrate from. Apex's migration guide here covered breaking changes
between two Apex releases (3.5 → 4.0); none of that history applies to a
codebase that started from the 4.0-era Apex source as its one-time reference
point.
If you are coming from the Apex original rather than an earlier version of
this port, you are not migrating so much as reading a different API entirely —
see salesforce-considerations for what carries
over and what doesn't, and known-issues for the full list of
capability gaps (record types, org seeding, a real Now persistence layer, and
more).
Once this port has its own tagged releases, breaking changes between those will be documented here.