The Albion No.9 moved from the Madejski Stadium to The Hawthorns last summer in a deal potentially worth in excess of £4 million.
Long told the Birmingham Mail: “I still have a lot of friends there at the club – not just the players but the manager, staff and higher up.
“I made a lot of friends in my time there and it will be strange playing against the blue and whites. I’ll still be doing my job, trying to get the winner and hopefully get West Brom the three points. It’s about business as usual.
“I won’t celebrate out of respect if I do score.
“Reading have done a lot for my career and I wouldn’t ever do that. I always gave my all in a Reading shirt, so hopefully they will appreciate that when I turn out against them.”