my take on it is to buy them, good ones, like NTK (don't start that China thing... I ain't buying your diatribe on quality..) and have them with you, in your bags, and when needed... simply do them.
Having replaced wheel bearings many times, (for myself, and others, and sometimes in emergency failures en-route on rides and rallies), I'll just say "replacing a fine bearing, may result in the new bearing failing in much less time, and at an un-opportune time", then just running till it's clearly "needed".
no guarantee on ANY life cycle, of any bearing, when a working one is pulled, and replaced, with another... just like tires.... they all can change ....