It is no big deal, I would just recommend making sure the seat post and seat tube are clean and lightly greased, and use a torque wrench to tighten the bolt each time you change seat heights, you could also buy a spare seatpost if she likes a diffrent saddle or has it in a different fore/aft position.

ditto the second seat post. Shouldn't be that hard to source. While the clamps are relatively easy, it's far easier to just swap post+saddle and have tape marking the spot to insert to.

Probably more important would be the reach. Bike sizes can be amorphous things, I find seat height to be a rather easy thing to achieve no matter the bike but proper reach+ is the more difficult thing. I'd bet that in addition to swapping seat posts you'll need to swap stems too (and possibly spacers)

on the point of seat height being easy. I'm 6'2 and can get my desired seat height on 54/56/58/61 sized cervelos.... but only the 58 and 61 gives me the reach i need and only the 58 gives me a low enough stack.

you've opened up a can of multifaceted worms :)

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36 kona qualifiers 2006-'23 - 3 Kona Podiums - 4 OA IM AG wins - 5 IM AG wins - 18 70.3 AG wins
I ka nana no a 'ike -- by observing, one learns | Kulia i ka nu'u -- strive for excellence
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I was figuring that proper reach generally only mattered for me. I would be wanting to use it to race/ride. She just wants to burn calories on the trainer.

An problem that I'm hoping to solve with me getting a tri bike :-D

oh geez, that makes this much simpler...

i'd still vouch for the separate seatpost and saddle tho.

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36 kona qualifiers 2006-'23 - 3 Kona Podiums - 4 OA IM AG wins - 5 IM AG wins - 18 70.3 AG wins
I ka nana no a 'ike -- by observing, one learns | Kulia i ka nu'u -- strive for excellence
Garmin Glycogen Use App | Garmin Fat Use App