Change of Sponsor (College)

[HASEENA]

I was granted LTR for HNC/HND of total duration of 18 months (6 months for HNC and 12 Months for HND) from HTS state funded college.in January 2011
I have completed HNC of 06 months in June and received HNC certificate in July ,on 5th of September HND Classes will Start
but Now I want to change My sponsor and the course as well .My questions are

1.What is the process If i want to Switch From HTS to A Rated Sponsor?
2.What is the time limit of Changing the course after completing part of the course .i.e HNC because I'm still looking for the colleges .
3.Do I have established presence in UK(for the purpose of low maintenance)
4.In what circumstances UKBA can curtail my LTR?