Shimano 8 speed to 9 speed

In article ,
says...
If I have a Shimano 8 speed now and want to upgrade to a Shimano 9-speed,
then what do I have to do?
Just 2 new STI shifters, a 9-speed chain, a 9-speed rear derailleur,
a 9 speed HG casette, and that's all?
Did I miss anything such as spreading the rear of the bike to make it fit
the 9 speed casette or something?
Please advise me? Thanks, Paul

Unless your 8spd derailleur is an 8spd Dura-Ace, you can use your old 8spd
rear derailleur.
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Alex