Will Changing the Max Memory Setting Invalidate Query Plans? I recently lowered the max memory from the default (unlimited) down to 20 GB. Does this erase the oldest queries in the plan cache?
Per Microsoft documentation changing the max server memory will clear the plan cache, as will changing:


access check cache bucket count   
access check cache quota
clr enabled
cost threshold for parallelism
cross db ownership chaining
index create memory
max degree of parallelism
max server memory
max text repl size
max worker threads
min memory per query
min server memory
query governor cost limit
query wait
remote query timeout
user options