Forward Ajose, 24, is Swindon's top scorer with 22 goals in 36 league games this season, having arrived following his release by Leeds in September.
Iraq midfielder Kasim, 24, has also been linked with a move to other clubs.
Asked if he was planning for life without Ajose and Kasim, Williams admitted: "Yes, I think that's a sensible way to think."
He added: "Nick has scored so many goals, I think any club would be interested in a man who can put the ball in the back of the net that many times.
"It's beyond me that Yaser is still playing League One football. I can't find the reason. I'm struggling to find (flaws) in his game.
"It's highly likely that Yaser will play either Premier League football or Championship football next season.
"All the boys are ambitious and they should want to play at the next step. To try too hard to try to persuade them to stay is probably the wrong way to do things.
"I don't like to try to stand in the way of a player's progression, once a player has got that in their mind that they're desperate to go and play elsewhere."
Swindon's survival in the third tier was secured on Saturday as they beat Chesterfield, and they now face Rochdale and Shrewsbury in their final two games of the season.
"We've got a chance to look at some players who we're maybe not 100% sure what level they're at, but we can allow them to show us," Williams told BBC Wiltshire.
"We will try to work on a few different things in training and see if there could be an interesting angle for us to experiment with next season.
"There are very few occasions in football when you have the luxury to try something."
Meanwhile, midfielder Jake Evans, 18, has signed a professional contract with the club.
Swindon Town are planning for life without star duo Nicky Ajose and Yaser Kasim, says head coach Luke Williams.