Taqlid

Your previous prayers and fasts and worship are valid as far as your intention was for the sake of Allah, and you did not violate any basic rule of worship.

Taqleed is acting upon the verdicts if the most learned scholar (Al-A’lam) who is considered by the high caliber scholars in the Hawza who are known as Ahlul Khibra أهل الخبرة.

Switching should be to the most learned Al-A’lam.

Wassalam.

Taqleed is obligatory to keep based on authentic evidence from Quran and Sunnah. The most knowledgeable Marje' is to be followed. The highly learned scholars in Hawza know who is the most knowledgeable Marje'. It is not an option between three unless all of them are in the same level of knowledge.

Wassalam.

In Quran we read ( So, ask those who know if you don't know ( Sura 21, verse 7).

'Many Hadeeths from the Prophet (SAWA) and Ahlul Bayt (AS) order Muslims to learn the religious rules from those who know and never act with out knowledge. The Prophet (SAWA) said: Ask those who know. ( Amaali al-Tousi). Imam Jafar Al-Sadiq (AS) said: People who don't ask the knowledgeable persons will perish. (Munyatul al Merced).

Muslim needs to know the right Islamic knowledge to follow it and be on the right path. Right knowledge can't come from lay people or from the parents or friends etc, but it must come from knowledgeable persons Ulamaa' who have spent their life in studying and researching the Islamic knowledge from Ahlul Bayt (AS). This following the evidence taken by Mujtahids is called Taqleed, without which your religious acts will be with out authentic evidence.

Wassalam.

Quran says: Ask those who have the knowledge if you don't know. فاسألوا أهل الذكر إن كنتم لا تعلمون Sura Al-Anbiyaa, verse 7. Obviously we can not ask a dead scholar no matter how high was his knowledge.

Mujtahid must be alive to understand the exact situation of any matter then deduct the Islamic rule of it from Quran, Sunnah, Aql and Ijmaa'.

Sunni Muslims follow dead Shaikhs like Abu Hanifa, Shafi'ee, Malik and Ibn Hanbal. Even Salafi or Wahabi sects follow Ibn Taymiyyah. All of them died before many centuries and never witnessed nor knew about our current issues. Following dead scholar does not fulfill the need of guidance at all because full guidance needs full knowledge about the situation which is with a living eligible scholar not with a dead scholar.

All Hadeeths asking us to refer to the scholars never asked to refer to dead scholars but living scholars who can be asked and who can reply after understanding the situation.

'Wassalam.

In every field of knowledge, reasonable persons follow the highly knowledgeable experts in that field. It is the logic of life that the less knowing must refer to the most knowing. It is in medicine, engineering, law, etc and no one objects on it at all. Why some people doubt it when it comes to religious knowledge? This doubt is illogical. Allah is Quran ordered to refer to those who are more knowledgeable.(Sura Al-Nahl, verse 43 and Sura Al-Angiaa', verse 7 and Sura Faaturm verse 14).

Following the verdict of the Marje' is the way for people to follow the authentic evidence derived by the highly learned experts in Islamic knowledge. Those who doubt Taqleed do not have any other way to know the evidence because they themselves are not experts in Islamic knowledge. Some of them follow less knowledgeable persons instead of following more knowledgeable which is type of injustice.

Wassalam.

No, unless you were following that deceased Marje' before his passing away and the living Marje' allows you to refer to the late Marje' whom you have followed before.

Wassalam.

We should respect all the truthful Ulama who are guiding people to the Way of Ahlul Bayt (AS) being the real way of the Prophet (SAWA) which is the Way of Allah (SWT). Even if you don't follow his verdicts and follow the verdicts of other Marje' of Taqleed, must keep respecting the A'lam. If the A'lam issues an order (Hokm), you must implement his order in any case. Verdict is binding all who do his Taqleed, butHokm (order) I binging every one including those who do Taqleed of other Maraaje's.
Wassalam.

You need to follow the witness of two of more of Ahlul Khubra أهل الخبرة 

Ahlul Khubra are the high standard of Ulama in the Hawza who know the level of knowledge of the Mujtahids and define who is the most knowledgeable among them.

'Wassalam.