quoting Morrígan, Marquise:

Also, I forgot that OHG doesn't have umlaut yet! That's kind of a major development.

If you accuse me of nitpicking, I'll have to plead guilty as charged, but: OHG probably already had umlaut, it just wasn't indicated in the writing system, except for a > e. (The fact that this is indicated makes it likely that /u/ > /y/ etc. also were present in the spoken language, the writing system just hadn't developed the means yet to indicate them.)