You have user choice right now, but you want to change it to recurring? Should be fine. Users already using recurring payments will continue working fine. Those that used single payments will subscribe as recurring on their next renewal.

So you want everything to be single payments instead of recurring? In that case you'll need to go into your gateway account (PayPal I assume?) and cancel all their recurring profiles. Then set all your plans to single payments. If you intend to leave it user choice I don't see why you want to cancel their recurring payments though.

It's Authorize.net, I assumed the renewal originated from CBSubs, please clarify.

CBSubs triggers the ARB payment, but I believe certain parameters are sent with Authorize.net on original purchase for it to expect ARB payments. I don't know how it works exactly as I did not create Authorize.net.

In subscriptions Some are Manual, Not autorecurring and others are Mandatory, Autorecurring with notifications. Can these fields just be changed in the database?

In general you can not edit a subscription and switch it to recurring and expect it to work. The customer needs to manually renew as recurring instead of single payments. Forcing to recurring when they purchase as manual in most countries is illegal.

I've been asked to do this, there have been some issues with renewal dates that don't make sense.

Change your plans to Recurring only. The next time a user that made single payments renews it will be with recurring and not single payments. That is your only solution. I'm not sure how you'd go about stopping all recurring usages; suppose you cancel them from Authorize.net end.