Originally Posted by mstateglfr

Always do these things after its been painted/coated because that is when the threads will need to be potentially chased and the head tube and bottom bracket will need to be faced. When paint is on the threads, bolts dont secure easily/well. When paint is on what needs to be parallel and flat surfaces, they arent parallel or flat.
It doesnt hurt to do these things before painting too, but they are presumably already in good condition before painting so they shouldnt really do much.

Im curious what you have on the bike now and what specifically you are upgrading to as 9sp mid 2000s Tiagra is multiple generations old at this point. It works great, one of my kids has a mix of 9sp Tiagra and 105 on her road bike and its dead solid, but its quite old to usually be called an upgrade.