One warning on upgrading or downgrading stuff:
T5 pieces need less foundation support than vanilla pieces, but more than AoC pieces.
T6 pieces need less foundation support than both vanilla and AoC pieces.
If you upgrade from AoC Faction stuff to T5, extended parts of your building (like a ceiling 8 tiles away from its foundation) may collapse.
If you downgrade from T6 to faction or vanilla, or from T5 to Vanilla, parts may collapse also.
Upgrading from vanilla to T5 or T6 and from AoC to T6 is always safe.
This affects all kinds of building parts, not foundations - the data about the foundation support needed is stored in each individual piece, not in the foundations.