Does your current plan setup use auto recurring payments? If not it's relatively easy as you can just turn off renewals and new subscriptions for your old plans and add price/duration options to your main plan then on renewal they can just upgrade into the main plan. If you've recurring payments it's a bit tricky as you need to cancel future payments, which can be easily done for PayPal and Stripe, but not so much for some other gateways.

Within your PayPal account click the gear icon then click Seller Tools followed by clicking Manage in the Automatic payments box under the Getting paid section. This will take you to their subscription management page. You can also try the below URL after having logged in.

www.paypal.com/cgi-bin/webscr?cmd=_merchant-hub

You'll probably just want to suspend the payments as cancelling might cause CBSubs to do a full cancel on their subscription.