I haven't seen any relevant info stating that LAME 3.93.1 should be used with the --alt-preset | --preset switches.

The LAME changelog just states:
* preset medium added to the dll interface
* fix for abr/cbr presets
* fix -q0 switch

Is is time to update or is the recommended compile still 3.90.2, and/or 3.92?

There are more knowledgeable LAME minds here than mine, but I believe that the collective view would be that insufficient testing has yet been done on 3.93.1 and is unlikely to be done. Most, I think, are looking towards Takehiro's work in 3.94 as the next milestone worthy of serious attention!

3.90.2/3.92 are almost certainly still the encoders of choice at the moment.

On the other hand there was no big complaint about 3.93.1, even if some have searched for defects.

Gabriel: Is --preset medium the switch (modified standard) you suggested a while ago? Or does it include any special code tweaks?

It is a modified standard preset. No exactly similar to what I suggested, but quite close.

Thanks, that's what I needed to know. I won't have to go updating all my zipped software.