RBS40V used in existing RBK23, do I need to downgrade FW for the RBR/RBS20?

I am looking to buy a used rbs40v to ne added as the 3rd satellite in my existing rbk23 mesh, but I Noticed that on the support site, the most recent fw version for rbs40v is 2.6.2.8, but my rbr20 and rbs20 run 2.7.4.24 as the most recent version.  Back in the days when I 

Do I need to downgrade my rbr20 and rbs20 to 2.6.2.104 (the only 2.6.2.x series) and stay in that edition forever in order to add the rbs40v? Or can this rbs40v be upgraded to 2.7.4.24 like its regular cousin (rbs40 without the alexa)？

Thanks a lot.