Languages are constantly shifting and evolving. Just like in normal evolution, isolation can cause linguistic evolution to increase. Sometimes events can spur evolution, whereas sometimes it can just happen. Other times, these changes can happen in the parent population and not isolated one, such as what has happened with American English and British English. (That's right, British English evolved more than American English!)
This is exactly what it sounds like; some vowels can morph into other ones. For instance, the northern cities of the United States are experiencing something called the "Northern Cities Vowel Shift". It makes some words, like "cat" sound more like "kyat." In theory, any vowel can turn into any other vowels, but generally vowel shifts shift, instead of jump.
Another type of of vowel shift is when one vowel influences the pronunciation of another earlier. In German, this lead to umlauts.
In any case, you need to consult a vowel chart to figure out what changes. You should check out a IPA vowel chart, because it's fairly standard. (Wikipedia's is pretty sweet, with sound files included.)
These shifts have been some of the great reasons why languages change. For instance:
(Indo-European)*Pater -> (Old English) Faeder -> (English) Father
P->F and D, S ->T are some famous examples of famous consonant shifts. This is why German Wasser and English Water are different words. (Whereas German Hand and English Hand are more-or-less the same thing: S->T, but H stayed put!)
Finally, sounds can simply be dropped from words. Someone saying the "k" in "knife" or "knight" would get bizarre looks today, but they were once pronounced.
Regularity Vs. Irregularity In Languages
One other thing to consider: languages tend towards conformity in spelling and grammar except when it would be easier. "When it's easier" it determined by the speakers as a whole. Consider:
- Color vs. Colour (Changes in spelling)
- Creeped vs Crept "Crept" isn't wrong, but "creeped" isn't wrong either! It's changing from irregular (crept) to regular (creeped).
A Good Starting Point
Wikipedia's article on sound change may help you figure out what sound changes are realistic and which ones are not. Linguistics is a detailed and complicated field, but this should give you a good starting point.