Not certain what you are asking.
The “new feature” to Upgrade Android version without loss of data really only applies to Upgrades from S → T.
In any case, essentials should be backed up before any significant job.
I seem to remember some users reported an Upgrade R → S where data was not lost in the Upgrade, however at that stage users were asked to make a full backup.
A search of the forum might find those reports.
You are likely to be one of the first if you decide to try R → T
I have no first hand experience, but provided an answer to a similar issue Motorola - Moto G7 Play - channel - Documentation Suggestions - #3 by aibd. Notice that in the “upgrade to T” pages, there is the inclusion of sideloading recovery along with the regular system image.