Replacing the bearings requires removal of the triple tree. There is a set on the bottom and a set on the top. Aftermarket sealed bearings are available. If and when I need to do this job again, I'll go the sealed bearing route. The bearings now are loose balls in cages. Something to consider is this, when the bearings are in a cage, they may be replaced with free bearings, i.e., no cages. One would use more bearings due to the lack of a cage, thus allowing the bearings to ride on a different part of the race. I have always used free bearings when working on bicycles. In addition to the benefits of having the bearings ride on another surface of the race, the elimination of the bearing cage affords more bearings. More bearings means longer life and more fluid movement. Keeping the bearings in place when going the way of eliminating the cage is more arduous when reassembling. If you are not versed in changing out races or even in repacking bearings, please find a mechanic who is. Although the headset isn't as critical as say a wheel, the result of not properly pre-loading then adjusting the headset bearings can be a violent headshake at speed commonly known as a tank slapper event. No fun.
You could simple inquire from your triumph dealer how much they would charge you. Or make the inquiry from any motorcycle shop as the headset bearings are all basically the same. Any good shop should be able to change out your headset bearings. The oem bearings are like 45 bucks for the uppers and lowers (bearings in a cage with races). Therefore you are looking at about $100 for the parts and most likely about two hours shop labour.
My motorsickle, Vera, had the headset symptoms your sickle has. Vera's steering would settle into a forward position. This condition lasted for oh say 20,000 miles give or take and several years. Not really a problem for our cruisers. The problem does beg the question, "Are the bearings properly packed?" At the least I would suggest repacking the existing bearings. If you pay someone to do it for you, then by all means have them replace the bearing sets. If you do it, simple repack and readjust. Another thought, should you repack the headset bearings, you might as well change out the fork oil in each tube, yes and upgrade to the progressive springs also.
Whew. my fingers are tired.
Bottom line, be more worried about proper adjustment and lubrication, than the fact that the headset settles in a forward position.
