If you have the range you need, leave it alone.
If the shifting ergonomics are within your realm, leave it alone.
If it works as stated above, don't fix it.

If you need an expanded range, upgrade.
If you like the modern shifting experience (and limitations), upgrade.
If it needs changes to be a regular rider under you, fix it.

I voted to "upgrade," 'cause that's what I do most often.
As many have seen by a recent post, I'm not always successful.