Guitar Pro 7 upgrade can be purchased directly from our website via this page.
You do not need to remove your former Guitar Pro versions prior to install Guitar Pro 7.5.
In the same way you do not need to have a former version of Guitar Pro installed on your computer in order to install Guitar Pro 7.5.
These are different products that can be used independently.
Guitar Pro 6, Guitar Pro 5 and Guitar Pro 4 users can use their Guitar Pro User ID and Key ID to purchase an upgrade rather than a full version.
If necessary you can retrieve these IDs from this link.