If they knew going into the relationship that there was a possibility that you could not accommodate them long term and were ok with that, so it was an informed decision to choose something that might be 'short term' verses seeking out someone willing to make the long term commitment to them .... than no IMO you are not terrible for than following through with that possibility ... I have done the same thing with part time clients in the past ~ their written contract stated that

"Full time enrollments have preference and priority over part time.The aforementioned part time schedule currently works because there is someone able to share the opposite spot and provide the full time income. However part time clients accept that if enrollment changes and there is no one to share the opposite spot their option would be to take the full time spot to keep service going or give up the part time spot to any interested full time client."

This way my ass is covered ~ I would advertize for someone to fill the opposite spot as well as a option for full time and if I found a full time person FIRST than they would have to choose to pay full time or give up the spot ... if the chose to take the full time I would keep advertizing for someone to share it in interest of fairness to them to get them back to only paying for what they need but I am not willing to forgo income on a client unless there is a HUGE benefit to me for doing so (aka I want a lighter load for a while or what not and the decision is mine to make not that I feel 'obligated' or what not).