The 28-year-old will add his firepower to the ranks at Warriors at the end of this season as he looks to achieve his aim of playing Premiership rugby after six seasons outside the top flight, that started with Rotherham Titans in 2005.
However, he admits at the moment his commitment remains with the Exiles and that means potentially lining up against Warriors this Saturday afternoon in the capital.
"I'm looking forward to the move to Worcester now it is confirmed," said Claassens, the brother of Bath star scrum half Michael.
"This is the best opportunity for me and if you look at the facilities at Sixways, I'm sure that anyone in this league would love to join the Warriors.
"I feel this is a step forward for me and I'm happy the move has come at this stage of my career. You don't know when opportunities like this will come along again so you have to grasp them.
"I like the stadium, the set-up and the way Worcester look to play. This is an ideal move for my career.
"However, the moves come in the summer and now my commitment remains totally with London Welsh. I'm sure the Worcester fans will understand that.
"We have started to play really well in the play-offs. The two back-to-back wins against Bristol has given us huge confidence and we have clicked as a team.
"It will be a hard game this Saturday, but we are excited about the Warriors coming to Old Deer Park again. Lots of fans will be at the game and it should be a fantastic atmosphere.
"I would love to play in the Premiership and I feel the move to Warriors gives me the best opportunity for that to happen," he added.
"If I can't play in the same team as my brother Michael it would be great to face him in the top-flight - I hope that will be the case in the coming years.
"We have always got on well off the field and we're not too competitive. But I'm the older brother so hopefully if we ever come face-to-face he would show me a bit of respect!"