rca conversion to hdmi

My setup is about 10 years old so there are no Hdmi connections.  The current boxes are the version that Comcast does not make anymore so when one goes out it has been difficult for the techs to find a replacement box.  The boxes I have now have been functioning fine and I was hoping to wait as long as possible before making a lot of tv/receiver/speaker purchases for replacements.  I have been paying for the extra sports package which I use mainly during football season.  Last night I was going to watch the PAC12 Network and a message came up that I needed to swap devices.  I checked a couple of other channels in the package and got the same message.  In talking with Comcast they said I needed to swap boxes to get the channels.  I am not sure how long this has been the case.  Is there anyway to correct without upgrading the boxes?  Also, if I get new boxes is there a way to use a RCA to HDMI connection box so I do not have to upgrade my equipment?  Is there a recommended box so everything so sound will match video without delays?