To answer the second question - you can accept your offer without doing either of those things, although you will need to do both before you start on the new course (and ideally you want to inform SFE well in advance so you can complete the usual application on time to get your loans if entitled to them at the start of the year). The major concerns would also be SFE related whether you withdraw from your current course now or later.
Firstly, if you withdraw now, then you may need to pay back some of your maintenance loan from this year. If withdrawing from the course you normally need to pay back pro-rata how much loan hasn't been "earned"/"used" yet in terms of how far you are into the term/year. So if you have had your first two loan instalments and withdraw the day before your third disbursement, you wouldn't normally have to pay back anything.
If you withdraw the day after your third loan disbursement, you would normally be expected to pay back almost all of that final installment (and if you were halfway through the third "term", then they'd expect you to pay back half of the third disbursement, etc). So depending on your rental contract and whether you can exit it early and whether you have received your newest loan installment and are in a position to pay it back, it may practically be better for you to stay in the course until the end of the academic year. However if you are on an interruption of studies and not receiving a maintenance loan (and particularly if you are not locked into a rental contract you may be unable to pay or break!) it's a non-issue.
Secondly, more generally yes, your SFE entitlement will be affected by your prior study when moving into a new course. Your SFE entitlement will in general be the length of your current current course + 1 year - the number of years of prior study in higher education (HE). So if the new course is going to be a 3 year course, and you've studied one year in HE, then your entitlement would be 3 + 1 - 1 = 3, so you would have just enough funding to cover the new degree.
Note though if you have been funded for any "partial" years where you interrupted or withdrew partway through the year, unless it was within usually the first two weeks or so of the year that you did that, your uni would have claimed at least part of the tuition fee loan and it would count as a full year in terms of your entitlement. Also note that they calculate this "backwards", so if you would end up with fewer years of entitlement than the length of the new degree, you would need to self fund tuition fees for one or more years and those would be the first year(s) in the new course. However you could still apply for a maintenance loan.