A good question. I don't think the sample size of (key) forwards moving mid career recently is overly large, but I found a few:
Buddy - jumped from 90 to 100 in his first year at the Swans. Never got back to his Hawthorn highs, and hasn't exceeded his first year Swans average (100 rounded in 2018 as well) ... so you couldn't really say he took time to adjust, if anything quite the opposite.
Tippett - had his career best year when he moved, albeit he went close to this a number of other times, both before and after.
Dixon - a tricky one. Averaged 91 in 2013, then three years of high 60s, including his first year at Port (2016). Then jumped to 91 again - was he playing the same role in 2016 and 2017? Hard to tell from the numbers, given he seemed to get better at everything, but I'd say it was similar. You could argue he potentially took time to get settled in, although his role seemingly changed in 2018 (more ruck, like 2013), so it's possible that his good scoring in year two at Port was a spike, or driven by better fitness, etc, rather than by progressively adjusting to a new club.
Betts - not sure you'd call him a key forward, but he certainly kicks plenty of goals and is another that moved mid career. Was an 80s player from 2010-12, then dipped to 67 in 2013. Moved in 2014 and bounced back to 79, then went 86 and 88 for his two best years. Had a good recovery in his first year at Adelaide, but got better again after that.
Ryder - a little like Betts (just taller!) in that he's obviously not a traditional key forward. His scoring has been up and down every season he's played since 2011 (which means he is due a good one!) but I think the goings-on at Essendon, playing a year at Port and then being banned, and the return from that time out, probably mean there is too much going on to read a lot into his scoring. Certainly didn't hit the ground running, but probably had a fair explanation for that.