I have no issues using Avast Premier, Avast Cleanup, or Avast VPN alongside Defender. The major change to Defender is though it's not enabled as the default provider, it isn't disabled either. Defender respects you've got another product but will ask for periodic updates and perform weekly scans by default. You can change this of course but since I've had no issues between Defender and Avast products, I would keep this ability enabled. It makes for a more secure computer, if Avast misses something, Defender will catch it (unless it's the boot-time scan, which is superior to all scans performed in user land.)