Upgrade them all the way to KX12 portal engine, then use the Kentico 12 Portal to 13 converter.
Definitely should ask the question of if it's needed/worth it, and what is being preserved. If they have a ton of content then usually this is still the fastest route, but if they don't have a ton of content and/or it's easy enough to rebuild from scratch, probably better that way.
We've already upgraded 2 portal sites to KX13 using the converter and in the processes of converting 2 more. The converter can preserve all your content, including migrate widget content, but in all of our cases the sites had a ton of content that starting fresh wasn't an option. hit me up if you need help.