Most citrus trees are grafted onto a different rootstock, usually sweet orange these days, even if the fruit from the top graft is going to be lemon or lime. The only way a lemon tree can completely turn into an orange tree is if the grafted part has died and the rootstock then grows on its own, producing the fruit its programmed to produce, depending on the rootstock used. That might be sweet orange (most likely) or sour orange, or even, on older trees, rough lemon. The grafted part might die for various reasons - infection or even over rigorous pruning, right down to the base, which is likely to encourage the rootstock to take over instead.
The other possibility for partial change is if a sucker arises from the rootstock (below the scion, or graft point, at the base of the plant, usually) and is allowed to grow - because rootstocks are generally more vigorous than the grafted parts, this may well grow strongly and produce its own, different fruit. More suckers may arise, and they take over the growing process, and the grafted part dies or fails to produce.
Chimerism is somewhat different - this might be something that happens in very old trees, but citrus stock has, for some years, been produced by cloning, which means chimerism, unless it occurs naturally (rare), isn't really an issue via grafting. Even if it did occur, it wouldn't make the whole tree an orange instead of a lemon.