There's a fair amount of considerations that goes into migrating to SDA from a brownfield deployment but I can provide some starting points:
You would need to start out by checking the SDA Compatibility Matrix to make sure your devices are compatible with your DNAC release and what role functionality you want to assign to your devices. Then, you can do one of two things: 1. Add an additional L3 routed access, access layer device to act as your Fabric Edge and then add more Edge Nodes as you scale (your case scenario since you're replacing access layer) or 2. Change your distribution layer devices to Fabric Edges AFTER you use the Custom VLAN # feature to mimic the VLANs that are on your existing network onto the new Edge Nodes (edit: and then dependent on security preferences you can move your edge devices back down to the access layer, then set your distribution layer back to the way it was originally).
Before you start the SDA process, you would need to configure the following on the proposed fabric nodes (not limited to): set MTU to 9100, ip routing, username and password, configure VTY and console lines for access, NTP, SNMP, Syslog, loopback interfaces because they're used as the RLOC in LISP, underlay addresses, optionally multicast if you want to run native multicast in the fabric.
It would be wise to check out Cisco SD-Access Migration Tools and Strategies – BRKENT-2079 if you have access to it. Also more helpful resources here: Cisco SD-Access Fabric Resources.
Disclaimer: Best practice is to always have a plan for how you'll do your ip addressing, VLANs, VRFs, etc. Having a migration plan takes precedence over everything else.
