Here's the rub: How well they are ranking. The better they are ranking, the more you are correct to not want to mess around with them.

This is always a vexing issue when changing platforms.

It's complicated, right? On the one hand, if you have a new look, you'd like to maintain your branding appeal and consistency of the "look & feel" across your site. This much can actually typically be changed with minimal impact up or down (though I'd want to see examples first before making an actual recommendation).

So, if some or all of these pages do rank really well, you probably won't want to 301 redirect them as that will force a re-indexing of them which could take a while (so in the meantime they could drop off entirely) and once re-indexed, they might not do as well in the content of your overall PC site theme. That is to say, there is no blanket cut & dried answer to your question here. It depends upon context.

The more important your ranking for any of these pages, the more I'd advise being very conservative (it should be possible to massage in the most important aspect of your new site theme while maintaining the good stuff about those pages that have them ranking well).

The less important your ranking (or maybe the less valuable the ranking because they aren't converting traffic to anything interesting), then by all means pick a few and 301 redirect them to PC content pages as Greg suggests. Test them over 2-3 months and see how the wind is blowing there.

Without actual example pages, though, it's not possible to make solid recommendations. Could you provide a couple top ranking pages from your legacy site and then a link to your PC site?

 

Once you have that in place, go take a look at what one can do with the web.config file.  You can create 301 redirects for any file name extension and point to your PC pages.