If your training goes from, say June to June, then in theory you'll get 50 days holiday - assuming you are given holiday at the start of each year. I carried over holiday from the previous year, so in my year of ATPL studies, I had around 35 days of holiday. If you're timing is really good, the brush-up courses will fall in the bank holiday week, thereby saving you a couple of 'holiday' days too.
People leave careers, change jobs, move on etc all the time; it's just a fact of life. My company have known about my aspirations for many years, and as long as you are working hard, and are a good performer, you'll find that most experienced managers will accept the situation. In my company, a few others have changed careers too. Someone went onto become a professional garden designer after a couple of years of study, another went to open a diving school, someone bought themselves a yacht and sailed the oceans, unfortunately being killed in the process. More recently, we had someone leave to become a photographer after several years of study.