Assuming your old Skyliner antenna is in good condition, realize that you will not improve your reception at all with a new one. In fact, the old Skyliner will actually have more gain than the new Wiengard. Antennas are not different for the type of signal they receive, digital or analog, so there is no such thing as a "digital" antenna.
But the old Skyliner antenna can get tired after a number of years, and has a lot of pieces and parts. Most of the really old ones used 300 ohm flat wire lead in too, which has become pretty obsolete vs. 75 ohm coax cable. Pulling a new 75 ohm coax in is virtually impossible in most Airstreams, if you think you can simply pull the old out and a new one in. Usually you will need to cut off and seal the entrance for the old cable and find a way to run a new one into the rig, say through the back of a closet. Often new cable is run down through the refrigerator vent.
Mechanically I don't think there is a lot of difference in the hole size from one to the other, but my limited experience is that removing the through the roof parts from the old Skyliner can be a bit frustrating. Often they are rusted/corroded in place and covered with layers of old hard sealant applied over the years.
So, that is my limited experience with your project.