order of nameserver upgrade?

[bob prohaska's usenet account]

Is there any rule of thumb for the order in which master and
slave nameservers are upgraded/modified? Slave first, master
first, or something else? Is it worth the trouble to set up
a host as both slave and master, selected via choice of named.conf
on startup?

Only two servers are involved, with only two routeable IP 
addresses. The replacement is motivated  by hardware age, 
not load. Uptime is nice, but few will notice outages 8-)
As you may surmise, this is an educational project.

Albitz and Liu discuss _adding_ servers, but I found little 
comment on the best way to replace them.

Thanks for reading and any counsel.

bob prohaska