You will not lose the changes if you switch from a Monthly Plan to an Lifetime purchase through the in-app purchase.
However it is a good practice to take a backup periodically and link it to Dropbox, so that your customizations are secured.
Also, when you switch from Monthly plan to Lifetime purchase, make sure you turn off the Auto-renewal of the Monthly subscription from the App Store / Play Store.
Related articles
Avaz FAQ: How to cancel auto-renewal of my subscription?
Avaz FAQ: How to take a backup?
Avaz FAQ : How to copy to dropbox?