I want to test a new modem but possibly go back to the old one. Can I do that without problems?

Technically, yes you can, but be aware of the potential problem you might encounter.  Your modem has an eMAC, a unique identifier that is used to activate and provision the modem.  If you add a new modem, your old modem will not be cleared from the xfinity controllers right away.  You will be able to activate the 'new' modem, but if you decide to go back to the old one, it may not activate until it has been cleared from the entire wide area system.  The old modem may continue to show up in your account until it is cleared; it can take two or three weeks for that to happen.  During that time frame, you may have problems activating it, even though is still shows in your account.  The auditing and tracking of that eMAC address might cause it to be identified as already activated when it isn't, or might show as being assigned to an account, which even though it is your account, can prevent it from activating.  On the other hand, it might all go smoothly.  There is no way of knowing until you try.  Is the 'new' modem a new one, and not used?  If it is used, it can sometimes be a nightmare getting it activated.  Many of the modems for sale on ebay, etc., are stolen modems, or modems that were submitted for return but still are assigned to an account somewhere.  If that is the case, xfinity will not be able to activate it. 

Hi there! If oyu are using a new modem, we will need to add it to your account in order for it to operate correctly. We can get this done for you! Can you initiate a chat and start with your full name and address? To start a chat, you must be signed in. Please click the chat icon in the top right of the page next to the bell, and send your message to "Xfinity Support". Talk to you there!