First you must deactivate the free version of the plugin.
Then in Plugins / Add new / Upload, upload the PRO version plugin install and activate it, you will see that everything works fine.
I recommend that you finally delete the free version.

Not all the data you already have is stored in the database and you do not lose any when you update.

The data is stored in the database with the extension _wpcm.
Remove this table from "phpMyAdmin"

Do not.
It is a single payment and this entitles you to possible updates and technical support by sending Tickets.

When there is a new version of the PRO plugin you will be sent an email with the download link of it.

It can be done by modifying the translation of these fields with the "Loco Translate" plugin.

You can also "Submit ticket"  requesting the name change of these fields.

New fields can be added in "Contact type" as explained in this section.

You can also "Submit ticket" requesting the name change of these fields and you will be informed by Email of the cost of changes.