The Monster Manual says you're out of luck. On Space/Reach says that
A template may change this [space/reach] entry if it changes the monster’s size. The typical space and reach for monsters of each size is given on Table 7–1: Creature Sizes (page 314). Note that this table does not take into account special situations such as a roper’s strands. (292)
Then the Monster Manual takes this a step further in the section Creating a Monster on Space/Reach, saying
Some creatures have exceptional reach due to a particular weapon choice or a quirk of anatomy. It’s all right to assign a special reach figure that applies to a particular attack (such as the roper’s strands). (301)
(These sections are referenced because the section on Size Increases (291) fails to mention reach at all!)
Somewhat fortunately, there is an example of a creature that possesses tentacles that have greater reach than the rest of its natural attacks and that sees its size category increased: the Monster Manual section Example of Monster Advancement (292) shows an otyugh going from 6 Hit Dice to 15 therefore from its normal size category of Large to its new size category of Huge. The Large otyugh possesses a 10 ft. reach normally and tentacles that possess a 15 ft. reach, while the Huge otyugh possesses a 15 ft. reach normally and tentacles that possess a 25 ft. reach!
So the only thing readers really know is that game's designers thought that a Large otyugh's extra 5 ft. of tentacle reach should become an extra 10 ft. when the otyugh becomes Huge. But it's a mystery if that applies generally and a mystery if a (somehow) Gargantuan otyugh's 20 ft. reach (as per Table 8–4: Creature Size and Scale (Player's Handbook 149)) should be 30 ft. with its tentacles or should scale to 40 ft.
The Monster Manual says twice that it's just not going to supply this information. That's enough for this DM to make it up himself. I recommend you do the same.