I did the same with my Rio 125, I bought the Juwel T5 light unit to replace my T8 one. Some members will say that the t8 is perfectly acceptable to grow plants under and I agree, it worked well for me. Though I think it was a great upgrade as many of my plants flourished with the addition of carbon dioxide and macro dosing.
possibly a few other members may agree but with t5 upgrade you may want to be considering injecting CO2 if you don't already and dosing macro nutrients but I would wait for some more experienced members to reply.
I will reiterate that it was probably the best "upgrade" I did for my 125, I am running the Arcadia JT5 HO plant pros currently.