Is it permissible to amend a plan at any time to change the stability period for determining lump sums from one year to one month? We have a plan that we would like to terminate and would like to take advantage of the recent higher 30-year Treasury yields.

You can change from annual to monthly stability period, but only if the prior approach is retained to establish a minimum benefit for twelve months (or more). In other words, it might not get you what you're looking for.

From the preamble to the 417(e) regs:
"A plan amendment that changes the time for determining the applicable interest rate will not be treated as violating section 411(d)(6) if each distribution made until one year after the later of the effective date or the adoption date of the amendment is calculated using the time for determining the applicable interest rate as provided before or after the amendment, whichever produces the larger benefit."

Thanks, Steve. This has been extremely helpful.