Glen was on loan at Millwall before returning to Upton Park in the second half of the season - and making an immediate impact at right back.
He was then sold to Chelsea for £6m in the summer and is being tipped to be picked for the full England squad in the not too distant future.
But he admits: "I am missing everything about West Ham - all the fans and all the players, definitely.
"It was a surprise the way it all happened so quickly but I didn't really think about it.
"I hope the fans do understand because I didn't ask to leave or want to leave.
"But the club can't reject that sort of money after relegation.
"Obviously I was upset to go because I had been there all my life, but some things just have to be done."
And that trip to Upton Park on Sunday?
"I might do, I have thought about it - with the fans in the crowd - and I would be supporting West Ham," he says.
As for the hype surrounding the game, he adds: "They have to stay calm, relax, and just enjoy it really. Over the years this fixture has gone off but it isn't like the old days any more so it doesn't have to be like that - times have moved on.
"Everyone was nice over there when I was on loan, the people were quality, and I was treated fine."
Glen reveals he could even have been playing in this fixture if an original plan for him to be loaned back to the Hammers had gone ahead.
"My agent obviously said that I was going to go back there on loan but Claudio Ranieri changed his mind so it was nothing to do with me - I wasn't offered the chance.
"I didn't turn it down, they didn't mention it to me. If it had been offered, I would have done it, without a doubt, though I don't know if the fans would have liked it.
"But you never know what will happen in the future."
As for his new life across London, he says:"I think I am coping and it is going well. I am enjoying every minute of it as I always enjoy my football."