Mortgage payments

My mortgage payments are split in to two: 18mos and 2yrs. I want to call the bank and ask if I can put an extra 100/wk on my fixed mortgage payments. My question is if my situation changes and I cant sustain paying this extra 100/wk, can I ring them again and say can I go back to my usual regular repayments? Also, when I am due to refix my 18mos , am I able to increase/decrease the extra amount I am putting in?

Thanks