Well, we don't have a sh!tload of M14's in storage anymore. They've been given away to other countries except for those required for probable current operations (i.e. SEALS, SF, and USMC DMR program). The rest were given away to the Phillipeens, the former Soviet Union states, and a host of other small countries as aid. Many were demilled under Clinton. Last I heard the actual number of M14's on hand was something closer to under 100,000. The spare parts for these guns is almost gone as well. Given away to the new users of the M14 and sold off for scrap or spares through the CMP. It is assumed that the government is holding enough spares for their own usage, but they can also have sub-contractors make spares as required, since it's not a standrad issue rifle anymore.
The fact is, if we were to go back to 7.62mm, we'd have to go produce a new one. There aren't enough SERVICABLE M14's around.
Ross