I would leave it myself since it doing so well already, why throw a wrench in it when it is working.

my 2 cents.

I would leave it alone. That big of a change may trigger some sandboxing for a while.

I am in the same pickle. We want to add some keywords in our Urls but the risk seems to outweigh the reward.

One client opted to rewrite the "numbered" urls to keyword/title urls.
A second client wanted to do the same but was afraid and didn't.

A year later both clients (approximately the same size/traffic) are still doing okay, but the SECOND (above) suffered ZERO disruption while the FIRST lost three months of full traffic and troubled serp results. Makes me wonder if there is a sandbox applied when G sees something that looks like "keyword stuffing" in url AND page done "overnight".

In general I advise clients "If it ain't broke, don't fix it." Also in general I advise clients to get it right the first time so you don't have to go back and do it right later.

In the OP's case... there's a seven (7) year history already established. Any major change now will result in a major loss of trust by search engines. I'd leave it alone... but from this point forward make all new pages/urls the way you wish to move. At some later point, after establishing the new url structure, it might be possible to transition without loss.

I agree that it's probably wisest to leave the existing URLs as is, unless there were other reasons besides SEO for changing them.

The SEO advantages of keywords in the URL are just not big enough to be worth the disruption of changing existing URLs.

Look for other things you can do to fine-tune the on-site SEO, and start there.

IMO keep the old urls as they are but re-structure the new ones to use the keywords if possible. And there are 2 things I would watch for. a) Having the necessary keywords in the URL, b) keeping the urls as short as possible. I wouldn't put year/month in the url. In general for SEO urls the shortest and more meaningful url wins.

Thanks all for your comments (and confirmation too). :-)

So maybe I will keep the old the same and only rewrite later posts and pages to new structure.