In its current state, the platform does not distinguish between pre-populated (from your book for record/database) & pre-populated (from a resumed session). As such, there is not an option/toggle you can use to say that only the initially pre-populated information, not the resumed information, should be disabled.
Having said that, like most actions on the platform there is a least one creative approach you could use to likely accomplish this business need.
Example of initial payloads when launching a form:
Under the Advanced conditions for these fields, you could then set the condition to:
"Disable this field IF disabled.firstName is equal to True"
Assuming you follow those steps, any saved sessions will include all of the data from above plus any new fields captured during the session. The difference, however, is that the new fields will not have the equivalent “disabled.fieldName” set to “Yes”, so even if those fields have the conditional logic assigned to them, they will remain editable.
This approach requires a little upfront work but will give you the flexibility to create whatever workflow you require.