I have inherited this problem...
Draytek 2820 with 3.3.4.1_232201 firmware that is hopelessly out of date and seems it would need two firmware upgrades to get up to the latest so that I can use the config file to setup the new 2860np.

Trouble is there are a bunch of servers and critical systems running that merely upgrading the 2820 firmware could potentially be catastrophic should it go wrong.
I cant find any older firmware for the 2820 to match the existing 3.3.4.1_232201 should I need to downgrade if the upgrade proves problematic.

Anybody have any experience with this?

Not such experience here ... but maybe some helpful infos for the 2820
Painful but you can take current screenshots for each page (settings) to remember
or write them down
Keep a backup file with current settings in the case you roll back to current firmware

To get back your current firmware here (3.3.4.1_232201)
ftp://ftp.draytek.com/Vigor2820/Firmware/
and look for the link Vigor2820 V3.3.4.1 AnnexA 232201/
ftp://ftp.draytek.com/Vigor2820/Firmware/V3.3.4.1/annexA/

+ 1 for the screenshots idea. That's how I went from a venerable 2820 to a 2860

Screenshots have already been done but what to do about the IKE Authentication Method
Pre-Shared Key ?

That nobody seems to have recorded anywhere...............ugh