That’s three presidents in about 48 hours.
This second change, ousting the PM who had taken over the presidential role, actually reverts to constitutional procedure, as Al Jazeera notes:
The council declared [former president Zine El Abidine] Ben Ali’s departure was permanent and [speaker of parliament Fouad] Mebazaa was sworn in on Saturday under Article 57 which stipulates that when the post of the president falls vacant due to his demise, resignation or total incapacitation … the speaker of the parliament … shall immediately undertake the presidential duties on temporary basis for not less than 45 days; and not more than 60 days.
The piece of legislation states that it is not permissible during the transitional presidential period to amend the constitution or impeach the government. And during this period, a new president shall be elected for the term of five years. The newly elected president may dissolve the parliament, and call for premature parliamentary election (in accordance with the provisions of Paragraph Second of Chapter 63).
In a case like this, a “transition” following the rules of the established constitution does not necessarily augur for real change.