The new system should start empty of content.

I disagree with you on that one. Sorry.

I really like the idea that PM is a continuous thing for over two decades. I'd rather spend the time to find a way to convert existing nodes as well as possible to the new markup.

This would also help with search functions and linking to old posts. Having the whole history of PM posts, with all functionality is one of the things that makes PM a great resource.

Also, the exact type of markup shouldn't be a big issue either way for most users, my focus would be on supporting a WYSIWYG editor by default.

Same goes with the existing user accounts, we should import them as is, possibly deactivating older, unused accounts, e.g. disable login but have a simple way of re-enabling on demand. This way, the usernames stay reserved for the people who come back to PM after a few years. It also preserves the history of the site. This includes monks (and their posts) who have passed away - i can't, in good conscience, just drop all that stuff and relegate it to some hard-to-find archive when we have a relatively easy way to preserve that stuff in the live system.

Do you mean a link in PM2 node source like Markup in the Monastery would make an external link from there to here?

No, i mean links from external sites to PerlMonks. Cool URIs don't change

I think this is something that is easily achieveable in my framework with a few hours of work, especially if we do import all the old posts.