It looks like you might have in mind the possibility of "downgrading" open access rights, to go from being more available to less available, and that can get tricky. If a copyright holder does nothing, then absolutely nobody is allowed to make a copy (which seems to be diametrically opposed to your intent). With the right kind of license, anyone can make a copy (precluding a payment scheme). The standard commercial approach is to ease loosen up the "do-nothing" plan by giving permission to copy in exchange for money.
It becomes a problem if you initially say "anybody can do anything", and then later add the condition "provides that you pay us". That restricts subsequent unpaid copies of version 1.1 with that license, but has no effect on copies of version 1.0 which have the do-anything license, which undercuts any plan for paying. This simply means that you need to be consistent in your license for a given work, and if you add the option for payment later on, that would be for works subsequently added to the catalog. Since "take freely" and "copy only in exchange for money" are polar opposites, at the conceptual level you should not try to reconcile these options. Instead, you shouldoffer two completely separate services.
In order for you to put the work on your server at all (with or without the pay option), the author needs to grant you some license. They can grant you a non-exclusive license to distribute copies in exchange for money, which would allow you to conventionally sell copies), and since the license is non-exclusive, they can grant another seller a non-exclusive license to do likewise.
Although only the copyright holder can specify the terms of the license, you as service-provider can stipulate what kinds of licenses will be tolerated in exchange for your service. As a starter, the license should grant you permission to distribute in some manner, for some period, you probably need an indemnification clause just in case an "author" is an IP thief and the real author tries to sue you (though beware attempts to collect blood from a turnip), or you could be sure that you follow the DMCA "safe harbor" rules. Beyond that, there are policy and intent questions that you'd have to understand, i.e. what are you hoping to exclude?