The ministry of defense asked NPOmash to extend the service life of the UR-100NUTTH/SS-19 missiles, so they could be used as launchers in the Avangard system.

The report says that the last extension, to 36 years, was done in May 2019, but as far as I can tell, this was done in 2014. There were no life-extension reports after that (which doesn't mean that it hasn't been done, though).

In any event, the previous extension was probably quite different from the upcoming one - it was about service life of missiles that have been fueled and deployed. The ones that will be used for Avangard are those 30 "dry" missiles received from Ukraine some time ago. An extension would still be necessary, of course, and my guess is that these missiles have quite a few years in them.