Munir,
The Quran talks of previous scholars writing things saying that they are from God when they are not. In Al-Bayyinah the Quran says that they did not differ except outside of the clear proofs that God gave to them. Most times 'min ba3di' is considered 'after.' But according to Classical Arabic it can also mean outside of. We can see this in Yusuf Ali's use of 'my absence' for 'ba3di' in 7:150. Most translators do not realize this use of ba3d, just as many do not realize the use of qabl for 'in the presence of' or 'before' in the sense of 'I am standing before you.' Many translators miss these meanings when they translate. I think compiling all of the passages on the subject will bring many things to light. Primarily that The Jews and Christians do have the teachings of Moses and Jesus (God bless them both), although may not all of it but they also have a lot of what is not their teachings mixed up with it. That is why God in the Quran continually tells them to follow what God revealed, because they use Torah and Injeel in a way that it extends to the teachings and words of those who were not God's prophets. For them to know what God actually revealed they have to only follow what is quoted of their prophets instead of stating that all of their books are from God or from certain prophets when those books do not make those claims. They also need to be following the most original of their scriptures. This would be the Greek Septuagint, the Samaritan Hebrew bible and the Aramaic scriptures NOT the Massoretic Hebrew text which the Jews rewrote after Islam, purposefully changing things when they translated these scriptures back into a later Aramaicized Hebrew. This would delete much of what they claim is the word of God. They also need to include the authentic apocryphal words that have quotes from God's prophets. If they compiled all of this, studying it in the original languages they would be on a much better and correct path. The Quran then comes to clarify what cannot be clarified even when they do this. They are supposed to accept the Quran and use it to as a guide to interpret the compilation of sayings of the prophets of God sent to them and and to clarify what is not clear in these quotations. As far as Christians and Jews directly converting, I think this should be encouraged among their children more than themselves. Despite that I think they can convert and leave their previous religions behind, especially if they see that their scriptures are telling them to do this. I think the issue of the Sabbath is trickier. It is advisable in my opinion that people who have paternal ancestors that are among the tribes of the children of Israel should continue to follow the Sabbath.