Change numeric string to alpha string in sentence

[calz]

How do i change a numeric string in a sentence to an alpha string from what the user give as input.
Example: I have 15 apples in box12.

output:

I have BB apples in box12.

The alphanumeric string must not be changed eg. box12 must remain box12

[48T7M4Gy]

Test the words in the sentence and only modify the words where all characters in it are numbers?