Some aspects of the conversion can, and should, be scripted. Some should be manual.
There is no reason to go through each post. Prioritize them: if someone comments on an old post, clearly it’s active, so edit that. If a post is getting a lot of traffic, edit that. Otherwise, leave it alone until there’s a reason to edit it, or you have time to do so.
(Also: crowdsource this stuff. Let trusted volunteers submit edited HTML, then drop it in as a replacement if it’s good.)
Some aspects of the conversion can, and should, be scripted. Some should be manual.
There is no reason to go through each post. Prioritize them: if someone comments on an old post, clearly it’s active, so edit that. If a post is getting a lot of traffic, edit that. Otherwise, leave it alone until there’s a reason to edit it, or you have time to do so.
(Also: crowdsource this stuff. Let trusted volunteers submit edited HTML, then drop it in as a replacement if it’s good.)