There is a document about that: https://www.nxp.com/docs/en/engineering-bulletin/EB810.pdf
According to that, migration shouldn't be too problematic:
In order to allow customers to migrate from i.MX 6Dual/6Quad seamlessly, we have the following compatibility requirements on i.MX 6DualPlus/6QuadPlus:
• Pin-to-pin compatible, no ball map changes
• No PINMUX changes
• No IP version changes other than the internal bus structures, GPU, PRE and PRG•No memory map change for existing IP modules
• All features supported on i.MX 6Dual/6Quad are supported on i.MX 6DualPlus/6QuadPlus to the greatest extent possible
• Software compatibility is mantained to the greatest extent possible