It is just that modern speakers sometimes feel that adverbs must end in "-ly" (for example, "often", "today", "now", "already".. oh, wait).
The word "fast" is and was both an adjective and an adverb. It makes a lot of sense. Some English adverbs indeed use the suffix -ly (coming from Old English -līce). However, "fast" is a really, really old adverb, so it is only natural it never had any "-ly":
- "Ferk fast to the fire," quod sho, "that flames so high; There filles that fend him, fraist when thee likes."
- Ye shall goo robbe, stell, and kyll, as fast as ye may gon.
(apparently, "quick", meant "alive")
On the one hand, one might expect "fast' to cease being an adverb in the future, since some speakers think it should not be. On the other hand, for that to happen the majority of speakers should actually stop using it as an adverb.
UPD: I did not dig it all that deep but I think "fæst" in Old English produced its adverb by adding -e (fæste). Then, of course, the final -e disappeared.
First, меняться and измениться have different aspects. :)
However, we can compare меняться and изменяться, which are both imperfective. There are at least two types of change to think about:
- when a thing experiences some "modifications"
- when a thing gets replaced by something else.
Меняться is a general word that can mean either of the two. Изменяться is only about the former.
Менять and меняться can also mean exchange—something изменять and изменяться cannot. On the other hand, изменять (but not изменяться, naturally) can mean "to cheat" on one's partner.
When меняться requires a perfective counterpart, you have to select an appropriate one. Обменяться is for "exchange", измениться is for "modification", смениться for "replacement". Поменяться works for physical (not abstract) exchange of goods and also as a colloquial alternative if you mean replacement and inner change. Finally, перемениться can replace измениться, though, to my ear, sounds quite formal in that meaning.
Another way to look at the difference between меняться and изменяться it is to remember that prefixes из- (ис-), воз (вос-), пре-, чрез- are of Church Slavonic origin, so they tend to sound a bit more educated or have a more abstract meaning even when there is no other difference. The bad thing is, this is only a general trend: Church Slavonic elements are a plenty in modern Russian. Many words are considered normal words, and native speakers are not generally aware of their origin. For example:
- глава = chapter; head (a person) (cf. голова "head")
- враг = enemy (ворог is nowhere to be seen)
- сладкий = sweet (cf. солод "malt")
- страна = country (cf. сторона "side")
- убийца = murderer
By the way, there is also a linguistic use for изменяться (not измениться). When a word of a certain class has different forms, you can state that as follows: Существительные изменяются по падежам и числам.
"Fairly rare" would be a better description. I do not know old poetry all that well. In contemporary Russian words like полон and ворог are only used to deliberately sound arhaic—and while I suspect it was the same two centuries ago, I am not sure.
All I can say, Church Slavonic брег, град, перст were easy to come upon. I know I encountered them in poetry. I only learnt about native ворог in the articles on Church Slavonic influences.