You can absolutely replace the bearings without removing the t-arms...I've done MANY of them like that. It's sure easier to just get the whole thing off and work on a bench, tho.
There are also spindle supports that have grease zerks in them now. Some of the reman folks are doing that. It's just a simple drill/tap, and put in a zerk. Problem with those is, most folks will over-grease, and blow grease out thru the seals....which damages the seals, and then you get to start all over even sooner!

 Food for thought.  When I did my T-Arms on a 72' I used the high pressure silicone gray colored grease.  Does not seem to be as sticky as the fiber grease I have used for many years, but it sure is slippery.  I packed the hub as full as I could and put it back together, no zerk.  Take it out of the car and work on the bench. MUCH easier and you can see what your doing.  The set-up tool is a good thing to have in the tool box, get one.