SC3K is similar enough in structure to SC2K that it can actually load SC2K city save files directly; SC3K being a relatively strict superset of SC2K in terms of available elements and their sizes certainly doesn't hinder that backwards-compatibility at all.

The only things that would actively break when you did this were related to maximum terrain steepness for underground elements (metro, water pipes), 90-degree rail connections would sever (at least, graphically), and hydroelectric power generators as they're the only power building not implemented.

The arcologies were also dropped from SC3K, but to be fair those were mostly just designed to be population boosters in SC2K- something that wasn't necessary in SC3K now that the simulator was allowed to keep track of more than 250 buildings (or at least, so the manual and the query tool say).

Earlier versions of SimCity 2000 are more limited in this regard as arcologies after the first 150 special buildings don't count towards the population count; the later versions do, however.