depending primarily on
(I) your budget and
(ii) speaker shipping logistics, AND
(iii) assuming that you are still enthusiastically married to the SNELL family of speakers; and you will be able to find an antidote for any contagious "upgrade- itis" affliction going forward,
THEN here is what I would do....
(1) Upgrade your existing main SNELL speakers (RF and LF) to the best model SNELL available , assuming there are other upgraded models to fill that niche. If so, then this move is an upgrade for your 2-channel audio.
(2) reposition your existing front speakers to serve as the "new" RR and LR rears in a 5.1 setup.
(3) Purchase one used SNELL speaker pair -- matched as close as possible to your rears in (2) -- available in the used market as highlighted in the prior post. This will provide your CC and a mothballed spare.