Your student finance entitlement for full time studies would be: Length of the degree you will be on/entering into in the coming academic year + 1 year (gift year) - the number of years in higher education previously.
If you studied 1 year in uni before, and are starting a new 3 year degree from 1st year, the calculation is then: 3 + 1 -1 = 3 years of funding remaining, so enough for a full degree still. If you were in a higher year of your current degree and/or had prior study at uni before starting the degree you are now leaving, then you may not have enough SFE funding to cover the full new course.