Did Luther ever acknowledge that his Catechism would require textual changes as language evolves? In the preface of Luther's Small Catechism, Martin Luther warns twice that people should be taught a fixed version of important church doctrine and that not a syllable should be changed (emphasis mine):

The honored fathers understood this well, and therefore they all consistently used one form of the Lord’s Prayer, the Creed, and the Ten Commandments. We should do as they did by teaching these materials to the young and the common man without altering a single syllable and by never varying their wording when presenting or quoting them year after year. […]
Second, after they have well memorized the text (of the catechism), then explain the meaning so that they understand what they are saying. Do so again with the help of these charts or some other brief uniform method of your choosing; adhere to it and do not change a single syllable, as said above concerning the text, taking your time with it. For it is not necessary to teach everything at once, but one thing after the other. After they understand well the meaning of the First Commandment, proceed to the Second, and so on, otherwise they will be too overwhelmed to the point of remembering nothing.

Luther must have been well aware that language changes over time; he insisted on having the Bible available in the common German people spoke instead of using the Latin Bibles that would have been more accessible to the people of a thousand years ago.
Did Luther ever acknowledge that his Catechism, or any of the other things like the Lord's Prayer, would need to have textual changes as well? At some point, the
Whether Luther expected language to evolve in the future is not clear. The introduction of printing, and the political situation, had led to increased standardisation of different German dialects, although it was Luther's Bible that really came to define the German language. The trend in his time, to which he contributed more than anyone else, was towards a common German language.  

In any case it was of fundamental importance to Luther that the Bible be in a language understood by the mothers in the home  the children in the street, and the man in the market place. Henry Zecher wrote:


  Luther, a relentless perfectionist who might spend a month searching out a single word, talked at length with old Germans in the different regions. To better understand the sacrificial rituals in the Mosaic law, he had the town butcher cut up sheep so he could study their entrails. When he ran into the precious stones in the "new Jerusalem" that were unfamiliar to him, he had similar gems from the elector's collection brought for him to study.


Luther said sometimes translating the prophets into German was like asking a nightingale give up its glorious melody and instead to sing like a cuckoo. He felt the poetry of Job suffered more from his translation than Job had from his comforters.  He didn't claim to have a perfect translation, and made amendments as late as 1545, the year before his death.    

This being so, it seems almost inconceivable that Luther would have forbidden any alteration to the text of his catechism if changes in the German language rendered them necessary. 

So, what could Luther mean by saying in the preface to the Small Catechism:


  "do not change a single syllable"


as quoted by the OP?.

The following paragraph is from the same preface, and sheds light on Luther's meaning:


  So adopt whatever form you wish, and then stick with it at all times. If, however, you happen to be preaching to some sophisticated, learned audience, then you certainly may demonstrate your skill with words by turning phrases as colourfully and masterfully as you can. But with young persons keep to a single, fixed, and permanent form and wording, and teach them first of all the Ten Commandments, the Creed, the Lord’s Prayer, etc., according to the text, word for word, so that they can repeat it after you and commit it to memory.


Luther was not saying that a particular form, his or otherwise, must be used. He urged pastors to adopt whatever form they wished and then stick to the same form when educating the young. By using the same form year after year they would be able to memorise and learn. 

He was not advocating that every future generation of German children be taught using the exact same words, for ever unchangeable. He was merely suggesting  that having started to teach a group of children using a particular form of 
words it makes sense to keep on using that same form of words, until they have learned them. Introducing a new version before they have learned the old version will merely make it harder to learn. 

There is nothing to say that their younger brothers and sisters, even, must learn the same exact words.