Well inquiring minds would like to know what the Soldier did? According the the regs the maximum punishment from a field grade article 15 are as follows:
a. extra duty for 45 days
b. restriction for 60 days (maximum of 45 days if combined with extra duty)
c. oral reprimand or admonition
d. forfeiture of one-half base pay per month for two
e. reduction in rank to E-1 (E-4 and below) or reduction
in rank of one grade (E-5 & E-6 only).
I couldn't find anything that specifically addressed the issue of a lateral promotion, but what I gather is the punishment is a reduction of grade, thus a reduction in rank which is associated with that pay grade i.e. E=1 PVT, E=2 PV2, etc. Also it is usually understood that a CPL is in a leadership position, most often promotable and will proabably make SGT fairly soon. It's not necessarily a bad thing when a CPL is made a SPC if he/she is no longer acting in a leadership role, it's like going form 1SG to MSG, you're not going up or down, technically speaking.