For purposes of devotion (C.R. 700.5) and mana value (C.R. 202.3), Old-Growth Troll's mana cost remains GGG. Old-Growth Troll's triggered ability changes only Old-Growth Troll's card types, subtypes, and abilities (C.R. 613.1d, 613.1f; see also C.R. 611.2e), and changes nothing else about Old-Growth Troll. (But compare Old-Growth Troll with Yedora, Grave Gardener, whose ability makes the returning permanent a "Forest land" but without a name or a mana cost, because the permanent enters the battlefield face down [C.R. 613.2b, 708.2a].)
Old-growth Troll devotion and mana value as an enchantment.