Whoa, dev_tech... Easy there big guy, I know that you are starting BMQ in a couple of weeks and all, but you don't have a clue what you are talking about. I tend not to post anywhere here (evident in my post count), but I felt the need to put some proper information here. First off, many people change trades throughout their career, either due to changing personal or work circumstances, for a new challenge, etc. And this does not make you a 'burden and a sh**tpump'. Many people don't know what trade they want when they join, or they think they do (the little pieces of paper at the recruiting centre aren't the same as actually doing a job). As well, as you progress in your career, you may see new options open to you, or jobs that you feel may be a better fit for your lifestyle as your circumstances change. As to it costing the taxpayer money? Sure it does. However, it isn't a lot compared to other government expenditures, and it is worth a lot more when you consider that you are keeping CF members happy in doing their jobs, therefore keeping them productive.
To answer the OP, yes, you can change trades, but it can prove to be difficult. There is sometimes mandatory service periods with certain trades that you must serve, otherwise you owe money back for training. Other than that, there are various different programs to change trades depending on where you want to go, but they all have minimum rank and time requirements, so it would be several years before you are eligible, and possibly several more to be selected. I work with a few people that used to be in the Navy, and after 10 or 15 years decided they wanted to be closer to home more often, and changed to airforce trades.
Hope this answers your question