I wrote a fairly lengthy comment on EE's blog about designing a plausible language shift in fiction and wanted to copy it here.
There's three basic types of language change:
1) A language on its own shifts its lexicon, sounds, and syntax, each being less likely in order, depending on several factors.
2) A language borrows tons of stuff from another language. This is almost always words first and then sounds and morphology later. Oh, sorry. Uh, word structure - case, plurality, tense, etc. The latter is on record as occurring (there's a case of an Aleut language, I believe, borrowing all of Russian's morphology), but that's quite weird.
3) A contact language, which forms a pidgin, which becomes a creole, which becomes a new language. There are several English-word-based creoles in the world including various places in the Pacific and Caribbean (and more). These occur most frequently in situations where people of various languages are all put into one place at once. Hawaii's creole was created by speakers of English, Japanese, Chinese, Hawaiian, and Portuguese brought to work on the plantations.
So, in deciding your language shift, first decide which of the above is closest to the social situation. It sounds like you have in mind a simple shift in English over time. Next, decide on a couple changes. The critical thing is to make it consistent. Is it just a single word change, a change in sound, or a more grammatical change? "Tells" suggests a grammatical change, hence EE's very first comment.
Another change which might be easier to get across in written language is more pragmatic. Choose a sentence structure that already exists in English but is less common and make it common. Yoda-speak is an example. With the force, you are. A Jedi, you must become. Place predicates at the beginning, you must.
Anyway, just some ideas.