Ritchie moved to Bournemouth for an undisclosed sum as the Robins seek new investors.
Last season's player of the year at the County Ground told BBC Radio Wiltshire: "I had great times at Swindon. It was brilliant when the gaffer came in.
"To get promotion was an amazing achievement. This year we've started well and look like strong contenders to get promoted.
"I leave with a heavy heart. I'll miss everyone there, especially the gaffer. I felt like he helped me a lot and made me the player I am today. I've got a lot to thank him for."
Ritchie added: "I am looking forward to the challenge at Bournemouth and hopefully get promoted.
"It would be good if it was both teams going up to the Championship."
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