What the reason was to change №
До 28.05.2018 — 2001; до 26.05.2018 — 7001; до 08.1994 — 2001 ?

This is a long story.
In the period 1992-1994 the trams in Antwerp were renumbered from 2xxx to 7xxx.
In 2006, the 7001 became the official advertising tram for the Tram Museum. Its history is complicated, but it is still relevant today: the head of a depot also sits on the board of directors of the Tram Museum.
In that context, the historical painting, read: the livery with which the tram was delivered in 1960, was chosen.
The numbering (7001) was of course incorrect, but it concerns a passenger tram and not a museum tram.
Every tram enthusiast hoped for a renumbering to 2001 but officially, as long as the tram is in passenger service, this could not and cannot be done.
Then a bit of history: the Antwerp P.C.C.'s still rode in common with the N.M.V.B., the former long distance tram company. The period was from 1960 to 1968 and this was only on one relatively short route. Pictures of crossings between the two companies exist, but are rare.
The suppression of the long distance tram was brought back into the picture exactly 50 years later, with indeed a meeting between an Antwerp P.C.C. and a historic long distance tram.
The number 7001 was inappropriate: therefore it was decided to renumber the tram for the occasion.

https://transphoto.org/photo/1114154/?vid=75063

Of course this was not discussed or allowed at the highest management levels, but it succeeded. A day later the tram got its normal number back...
The same applies to this photo. The tram wasn't planned to drive at all, it was also in the wrong depot, but thanks to the right connections the car could miraculously (...) drive on the right line on Sunday...