Indexes on VFP tables--when do they get updated

Question for you legends of the Fox (data):

Say Table1 has "X" number of indexes. Program code does something like the following:

IF Condition1 THEN REPLACE Field1 with SomeValue in Table1 ENDIF

IF Condition2 THEN REPLACE Field48 with Datetime() in Table1 ENDIF

Are each of the "X" indexes updated for each replace, even if those indexes do not include any reference to Field1 or Field48, or only indexes that are affected by the data value change?

I understand that ADDing and DELETEing records updated all indexes; I would think the UPDATEs--regardless of what fields updated--would cause index updates for every indexed field on every update as well.

tia, --Michael