If this is a “new relevant idea” rather than the original idea that attracted subscribers years ago, the “new idea” is implicitly not what they signed up for. It’d feel like spam to me, which is not a first impression I’d want to make. It’s also a false impression of the 20k that’s driving the desire to resurrect the old account since they don’t actually represent your current supporters. 20k with zero engagement might as well be 200k with zero engagement – they still amount to zero engagement.
Someone else recommended double posting. I agree. If the targets are similar enough, it seems fine to post in a new account and share/recommend in the old account. If this drives re-engagement, good, if not, you have already started the new account. Maybe you can periodically send another share/recommend on the old account, but I’d limit the use.