Since you asked, for geometry maintenance purpose your suspension fork's axle to crown distance should be measured with sag (i.e. with your full weight on the bike).
The need to maintain geometry assumes you are perfectly happy with the current steering sensitivity.
Actually to maintain the same steering sensitivity the trail of the fork has to be maintained, and trail is affected by fork rake (otherwise known as offset) and head tube angle, while head tube angle is in turn affected by fork length.
If you want to be precise and are interested in the maths you can read
this,
this and
this!
If not you can just trust me that a 10mm increase in fork axle to crown length will reduce head tube angle by approximately half a degree which in turn will increase trail by approximately 3mm. An increase in trail will make steering more stable/sluggish, a reduction in trail will make the bike more nimble/jittery.
That is assuming rake is the same for both forks; if not an increase in rake by X mm will reduce trail by approximately X mm.
In other words, steering sensitivity can be maintained by a combination of axle to crown length and rake, e.g. a 10mm increase in length combined with a 3mm increase in rake will end up having minimal impact on steering sensitivity.
Then the next obvious question is how significant any amount of change in trail is? This is an impossible question to answer because it depends on how sensitive you are to change. In terms of figures as Dave Moulton said bikes used to have
zero trail in the old days, nowadays trail tends to be around 60mm for rigid forks, and around 80mm for mtb suspension forks (larger to ensure safe handling is maintained when compressed, which in turn gave rise to exotic/cult suspension fork designs like the Girvin Crosslinks which maintain trail when compressed).
Imho if a rigid fork delivers a trail between 50mm and 70mm, you are unlikely to be too unhappy. After all,
other factors also affect a bike's steering.
Hope it helps.