B. Changing the data type of a field used in an Apex class from number to text
"You can’t change the data type of a custom field referenced by other items in Setup such as Visualforce pages, Apex code, processes, or flows."
This would require the extra step of removing all apex references, etc. and would therefore also be a problem to consider.
C. Changing the data type of a field used in lead conversion from number to text
"You can’t change the data type of any custom field that is mapped for lead conversion."
A custom field mapped in lead conversion would need to be unmapped, converted, then mapped again. Again, a problem to consider.
NOT:
D. Changing the data type of a field used as an External ID from number to text
"If you change the data type of a custom field that‘s set as an external ID, choosing a data type other than text, number, or email causes the field to no longer act as an external ID."
This conversion causes no issue with the external id field acting as an external id field, so there is no problem to consider here.