If party A purchases a software license from party B, and party B sells all of its rights relating to the software, what happens to the license party A has? Must the new owner still honer the agreement? When party B sells the rights, must they inform the purchaser that such a license exists? For example could it happen that party C, the new owner of the rights, finds party A using their software and considers it stolen, but then party A is able to prove they received a license from party B. What's the best way party A can protect itself?
The overwhelming majority of software licenses are non-transferable. The buyer would need to take out their own licenses with the software owner.
For those that are transferable, the contract selling the business should address this.