It would be interesting to do the maths and see how many bulb changes it would take to make back the cost of replacing the fittings with standard length ones...
You can get a dual watertight 54W T5 ballast for £36-£60 depending on brand (prices from thelivingseas.com) and bulb prices would be dependant on whether you want to get generic lights at about £3 or something aquaria/plant specific <£20
A quick google found the replacement juwel bulbs on aquatics online for £25 a pop.
So if you were to do it on the cheap with an interpet ballast and generic lamps it would be cheaper to replace the whole light fitting than to replace just the bulbs.
If you want the arcadia ballast and generic lamps then you'd have saved money the next time a lamp dies.
if you go for the cheaper ballast and the specific aquaria lamps then you could find it cheaper after changing about 7 lamps (less if you get the cheaper aquaria specific lamps).
if you want the arcadia ballast and specific aquaria lamps then you wont really see any financial gain over the next few years.
I seem to remember reading various threads that state the the lamps you choose should be picked mainly on whether you like the colour rendition they give since it's all the same to the plants. Can someone either agree or dispute that for me as I can't remember where I read it.
These are just the results of a 10min speculation, you can probably find everything cheaper if you scout around a bit.