I don't see how they can. When the results are revealed to Glen he will automatically assume that Isaac is his since he thinks that is the DNA that was tested. Lexie will have to give up JT in order to save Isaac from being taken as Glen's son. In giving up JT, Lexie will reveal the baby swap and then the Pandora's box will be open. People will start asking questions, wondering how this happened and then wondering what happened to Hope's real son that everyone knows was born alive and well.
Secrets have a way of coming out on soaps even from dead people sometimes.
I think that this secret will be out before summer and so many of Salem's citizens lives will be changed forever because of it. Used under License.