Why not just keep the expired ones alongside the new ones you purchase and
are up to date? That is what I did.

Yep thats what I am doing .. to a point of course as there is the time
when the expired flares will take up all the space in your boat

Is it illegal to have expired had held and gun fired flares on board when,
at the same time, you have a full compliment of "fresh" ones meeting USCG
regs?

Its legal. Carry whatever you like in addition to the required non
expired flares.

I rather carry 6 expired SOLAS flare than 3 non expired regular
(=useless) flares

matt