Changing from prior yet to current (or vice versa)

[BG5150]

Where in the code does it address changing from prior year method to current (or vice versa)?

Or was it a Rev Rul?

Can someone point me in the right direction?

[Belgarath]

There are different rules depending upon which direction you are changing. You could take a look at IRS Notice 98-1, and regulation,1.401(k)-2©, IRC 401(k)(3)(A), and 1.401(m)-2© for starters. There's also an "anti abuse" rule for multiple changes - see 1.401(k)-1(b)(3) and 1.401(m)-1(b)(3). And slogging through these will undoubtedly lead you to some additional sections.