How to change values of a dynamic set of attributes?

If you know the list of attributes before runtime, use a Parameter as input in the StringReplacer.

If the attributes to be StringReplaced are defined during the run, I think I would try to fix this using Python. This is probably the easiest and best performing alternative.

If you want a pure FME solution, you could explode the attributes to features (AttributeExploder), do the selection of attributes based on _attr_name and the string replacement on _attr_value. Then recreate the attributes using an AttributeCreator using @Value(_attr_name), _attr_value. Then merge the attributes back to single records using an Aggregator. But exploding attributes to features can be expensive.