Network ReDesign - Need Some Advice

Hey everyone, hope y'all are safe.

I have been tasked with a very intensive network redesign for a new client (about 100 people) that let their IT team go. Either they were let go because documentation was shoddy, and nothing was kept up from an administrative standpoint or the management team made a huge mistake.

Due to inadequate documentation, there are servers and network devices that we do not have access to. The issues have been no passwords documented, no IP information documented, ACLs restricting access, etc.

As we introduce new hardware, and look to migrate existing networks to newly created ones, we are worried that communication will be lost between hosts and servers, servers and storage array's etc. We have currently found about 70 different VLANs and are looking to reduce that to about 10.

My thoughts were to create the new VLAN's and add the existing VLAN's on the switches so that we don't necessarily need to re-IP everything at once. This will allow traffic to flow, as we work to make the IP changes on servers and other hardware over time.

I was hoping that some of you more experienced gurus could provide some insights or gotcha's that you have experienced when working on this kind of project.