@moji
I have no idea what visa you can change to as I don't know which category your personal circumstances fall into.
Having entered on a tourist visa (Schengen and Schengen visa waiver) you can't stay beyond 90 days whilst you apply for a resident's visa. Only those who have entered Portugal on a resident's visa (D2, D7, study, etc) can stay after their initial visa expires, but even then the extension is limited UNLESS they have an appointment scheduled with SEF.
Those with a D2/D7 etc have already passed a number of the conditions applicable to them becoming resident, had they not the visa wouldn't have been issued. This system is in place to prevent illegal immigration by persons entering the country on a tourist visa.