Another Multiple Plans issue

I have it set up so that users can have multiple same plans - but how do i add additional ones (since plans only show at registration)?
I tried using the URL above the plan in the admin but that simply shows them the one they already have. Do i need to add additional plans via a trigger?
Plans are set up as a Base Plan with 3 upgrade options and i need to offer the user the ability to have additional base plans (each with their own upgrades). I cant set them as separate base plans because that would limit the number of plans they can have to the number of plan sets i made.
Any help greatly appreciated

To allow a user to subscribe to the same plan twice you need to enable such functionality on the workflows tab (last parameter on the page). However, for them to subscribe multiple times they must first subscribe once then resubscribe after previously subscribing. You can't subscribe to the same plan multiples time at the same time.

As always - thanks for the speedy and informative reply reply Kyle
Question for you - since we can make a URL to view a plan is there such a thing as a URL to purchase a plan that carries with it the plan that it is to replace?
e.g. /user-profile-cb/pluginclass?user=xx&plugin=cbpaidsubscriptions&cbpplanN[0]=xx&cbpsubscriptionid[xx]=x,xxx
Effectively sending the user to page to purchase a plan that will update an existing plan??
Any help greatly appreciated.

A plan URL is directly to the plan in question. If it allows upgrade then it'll present it self as an upgrade. Aside from that no there's no other URL structure that I am aware of.