How much can we change our bike wheel diameters before needing another bike fit? How much can we change our bike wheel diameters before needing another bike fit?

If we change the size of our bike tires, it raises or lowers the front or rear of the bike. That means it can affect the reach, handlebar height, setback, saddle angle, and handlebar angle. That means even after tilting the seat up or down to make it flat, we need the setback readjusted, and we likely need a new stem to readjust the reach and handlebar height. Adjusting the setback affects the reach.

It seems like even a mm of adjustment can be significant in terms of optimizing our bike fit. If we extend our knees, our hamstrings are engaged when extending our hips like on the downstroke but if our knees are bent, they don't get activated so we get less power. Most of us notice the stretch in our hamstrings when when trying to reach our toes with our knees extended. There's a reason why a bike fit can take at least 2 hours. Some bike fitters recommend another bike fit when we get new shoes as they can have different sole heights in addition to needing the cleats adjusted.

Sometimes we need or want to change the diameters of the wheels.

Examples include:


Tire clearances are different for both the front and rear wheels while the rider desires fitting the widest tires possible. Adding a fender also decrease tire clearances.
Even replacement tires with the same width markings can have slightly different actual widths when installed onto the rim.
Rim replacement since changing the rim inner widths can affect the tire widths.
Spare wheels since some cyclists have multiple wheels for different conditions, race days and training, different gear ratios, etc.
Upgrading the hub to a hub dynamo, internal hub gearing, coaster brake, power meter, etc. The new hub may have different hole counts so a new rim, which may have a slightly different inner width may be needed. 


How much can we increase the difference in the two wheel diameters before we need another bike fit?
The thing about bike fit is that it's almost exclusively about relating your body position to the mechanical parts of the bike. The relationship between handlebar, saddle and pedals is unaffected by changing the wheels.

If you make a change to one wheel diameter it will be equivalent to the tiniest of slopes. Let's take a worst case: Say you go from 35mm tyres to 25mm on the same rim (a 10mm difference in nominal radius, which is what actually matters) on a bike with an unusually short 1m wheelbase. That's a 1% change. Does your fit change on a 1% slope? More typical changes on more typical bikes  would work out closer to 0.2%. I doubt you can even measure whether the saddle is level to that precision, for example, never mind whether it should be. You'd have to change the bead seat diameter for it to make much difference, and while people do put 650B wheels on 700C frames they normally do both at once. 

Shoes are very different, in that they come between your body and the bike, and in a particularly important way.