There are a number of posts about people who had 100s of 1000s of miles expire. In very few cases AFKL were willing to reinstate them. So if you want to extend, better do it before. It's not like AFKL have a bunch of temps come in every few weeks to sift through dormant accounts, it is computers these days....

If you have enough miles for a few promo awards, you should also be able to book a regular award, which is always better than letting the miles expire. 45EUR change/cancellation fee just in case you want to 'redeposit' after making a qualifying ST flight might also work (NB I have no personal experience with this).