Well, if the 14-30R's were properly installed, they will be on 30A circuits. The 14-30 configuration isn't intended for use on 3 phase, either. It is intended for single phase 125/250V, 30A with a grounding pin. If it is providing 208V 3 phase, there is either no neutral or no equipment ground available. The other possibility is that only 2 of the three phases are available.
The L21-20 is a 5-wire configuration, while the 14-30 has only 4 contacts. Assuming that all 3 phases and the neutral are present in the box, then the L21-20s should be able to be installed (the center pin needs to be grounded to the box), but the breakers protecting them will need to be replaced with 20A units. If only 2 phases or no neutral conductor are present in the box, your electrician will need to bring a new conductor from the panel, which could be relatively simple (if the run is in conduit) or extremely difficult, depending on the installation.
As with most questions, it isn't possible to give a really accurate answer without looking at what you have installed there.