Hello @Antoine Leca ，
Thank you for posting here.
I viewed the CA environtment in my lab, 2012 R2 domain and 2019 CA server ( 2012 R2 domain and 2016 CA server), after duplicating the certificate templates, by default, the key length on the certificate templates are 2048.
For CA migration, if the CA environment after CA migration is healthy (we can check by opening PKIview.msc console and all the status are OK), the PKI instructure should be fine.
For key length on the certificate templates, we can change key length when we duplicate certificate template.
Or we can change key length when we request certificate through MMC console.
We can chang the key length on the certificate templates if needed though the key length on the certificate templates changed mostly from 1024 to 2048.
I am sorry, I can not find instructions available to update the key length.
Hope the information above is helpful.
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