I have changed my home university/polytechnic. Can I keep my present user account and join it to some research project under the new university department?

Yes, you can, and the move is actually obligatory so that the university user statistics will go right. Find out if your new 
professor/project leader has suitable on-going projects at CSC you could join. If not, then we can open you a new research project. Give the transfer details and project supervisor info with the application form for computing services. More information from the CSC UserManager.