We switched without warm-up.

Its the reputation of the ISP (ASN) thats important and we didnt experience any problems when we switched.

I recently switched my main SMTP IP (2 months ago) and also did not do anything like a warm up.  Actually didn't even know that was a thing before the recent threads here.  We did not have any problems either.  Just made sure DMARC/SPF/DKIM, PTR and DNS were all sound when we cut over.

SPF/DKIM is ultra important regarding reputation and its paramount to have that setup from the beginning since you will end up on spamlists and the reputation is fucked.

When I made the switch to a new provider and public IP addresses, I set up a non-production domain on SM and exchanged email with my google email account after having set up SPF/DKIM for the domain. It worked immediately, and google was happy with my new IP. Next I went to my DNS configuration and set the TTL for the various records that were going to change to a 1 hour life and let that propogate. Over a weekend I then moved over my production domains after making the appropriate changes to the DNS information. It worked well for the most part, only had a couple of folks get hung up behind DNS servers that didn't honor the TTL value. After things were working I went back into DNS and set the TTL to the default values.

The warm up is not a general rule.

Let's say that the problem is not the reputation of your ISP but the arrogance of some large providers such as Outlook, Yahoo, Google.

When they start seeing messages arriving from a new IP (depending on the volume obviously) they assume guilt. It happened to me twice, despite the reputation of the new IP verified with Talos, Cyren etc. it was excellent.

They block because he is presumed guilty.

Then open a ticket. First automatic response from the system that gives you generic information. Insist following the instructions in the email by escalation and generally within 24 hours you will be resolved and they will definitively unblock you.