Look at what a major, and I mean a MAJOR refit involves.
Say you want to replace the engines, the electronics (FCS, ECM/ECCM and sensors) as well as the main battery.
First, you have to pretty much rip the ship appart to even get at the parts you want to replace. Then you take out the engines and put in the new ones. As those have more thrust, you have to put in more support struts as well, which plays merry hell with space allocated to other parts, but you finally manage it. Now out comes the electronics and in goes the new one, but oh, you need new wiring, because the old one worked with 500 Mb/s I/O speed and the new, improved on needs a 2 Gb/s which the old wiring can´t handle. After months of crawling through every tiny space in order to replace the wires, the job is finally done. Next is the main battery. The old 150mm lasers are replaced by the new and shining 200mm. But those don´t fit into the armored weapons bays, so those have to come out too. The new bays don´t fit between the support struts, so new ones have to be designed and fitted, finally the new guns went in. Oh, but the power plant is not sufficient anymore, we need a new one! The new plant also reqires new wiring,...
And this assumes, you at leest keep the displacement as is.
To make a real world example:
Because of the London Treaty, which forbid the building of new battleships between WW1 and WW2, many navies refitted their BBs (most notable I belive, were the japanese and the italian). In terms of both, time and cost, building new ships would have been a lot more efficient.
I remember only one case, where a deliberate increase of displacement was tried. Germany around 1900 cut it´s coastal armored cruisers in half to add a 30 feet section, thus increasing length and displacement. It was very expensive and took a hell of a time - not to mention that it was a total failure.
Edit: I forgot: But I agree, tripple time seems excessive.