How can we still download our software that we had bought though the Ubuntu Software center? Since now Ubuntu uses the the Gnome Software by default and there is not any apparent ways to reinstall the old Ubuntu software center.
Gnome software center and Ubuntu software center are just software to allow you to download and install other software. The software it shows will be the same in both (well, there will be different libraries). 3rd party software is likely to be available in gnome software center too, maybe you need to open the "partners" repository to get it shown?
(you did not tell what the software is; add it to the question and the answer is likely to be more targeted to your problem).
- The old Ubuntu Software center is listed in the 17.04 package list but it will install the 16.04 package since there is no newer version. Direct download to DEB file. That probably still works (have not tried it ;) )
Regarding paid software: you also get support from the seller of your software. And they are likely to have their own repository, their own debian installer and their own download site.