Well, you have basically 2 issues to deal with: a different temperature coefficient, which should not be a problem since you want to use laterals which have a negative one (self-protecting) in contrast to the positive of the verticals.
However a circuit for vertical mosfets will have some sort of temperature compensation, like a junction (bipolar transistor). This compensation will continue to reduce the bias current when the amp warms up. This you would need to manage.
2nd the transconductance of the laterals is usually lower and for source-followers this would not be any problem at all. As far as I know at least the Aleph3 uses the mosfets in common-source mode, so you get a reduce voltage gain.
If this is a problem in your setup, I do not know.
However I do not have any practical experiences with such subsitutions, so I can only advice you strongly to wait for advice from more knowlegeable people.
All the best, Hannes