I have a site that is using a very old (maybe 5 years back) version of K2. I just upgraded to joomla 3.8.7 and had a problem with the K2 logon form (captcha) so I would like to upgrade but just need an opinion regarding potential problems of going to a current version of K2.

Thanks - Mike

Which version of K2 of are you using?
Also are you using overrides and 3rd party K2 extensions?

I was on 2.6.8 and bit the bullet, upgrading to 2.8.0 after getting current with joomla 3.8.8 - everything seems to be functioning just fine.

2.6.8 -> 2.8.x will work without any modifications (unless your overrides contain really specific code).
Just make sure that if you are using any 3rd party K2 extensions, they will also work with 2.8