Yeah overall, although it was most definitely a pain and not something we were remotely relishing, the outcome has been worth the hassle.
Most of our key people (i.e. most engaged members) re-subbed straight away, then handfuls after further reminders to our old list. Roughly 25% re-subbed on first mention, then further batches of 5% p/month or so over the last few months.
So yes, although we’re roughly half the size of our old list now, there’s much more engagement - & it does now show how antiquated some of the old list’s data was with many on there - but open rates are the key thing - it’s gone from roughly 30% open rates to around 55-60% open rate.
(Although a while back I did read something to suggest that Mailchimp’s open rate figure isn’t totally accurate, and it’s most likely more than stated - something to do with people either double-clicking an email registering an ‘open’ as opposed to opening it with with one click, with the email’s content in the preview window on right hand side etc - it can somehow skew the figures…)