hi

Most important and only thing you have to do after an upgrade is to update the statistics of your databases with

the command sp_updatestats on all databases.

no big issues unless you load the temp tables or temp variables with huge amount of data.

if loaded heavily then convert those temp tables into a permanent temp table or...

Please make me understand the point that sql server takes parallel plan sometimes and serial plan sometimes even when the query is same and the environment is same.

some times think...

parallel plans are decided by all and mainly available CPU and Memory to complete the query and so the time wen u ran the query there may not be enough...