dnd 5e – Would it be groundbreaking if a character of several classes had a "single" class?

This is called "re-spec" and is sometimes useful.

While changing the character levels in the 5th edition of D & D is not a standard rule, it is not unreasonable to allow a character to change a badly built character once.

This is commonly known as "re-spec" or "rebuild," and I've seen it put to good use in previous campaigns I co-starred in. Most of our gaming group was new to D & D at the time and many players made mechanically very suboptimal decisions. When we reached level 10, the DM allowed us to change our characters only once to make sure that no one was bothered with an ineffective character early on for lack of decision-making.

It is not unbalanced, but should be used sparingly, for example once per campaign. It must be clear that this will not be a normal thing, but a one-time solution. If you allow frequent changes, the characters get too much flexibility (for example, they can completely change their character based on the adventure) and affect the continuity of the game.

You may want to allow other players to create a new specification at the same time, just to be fair. However, it must be made clear again that this is a unique grace that players will not expect to be repeated. As a DM, it is your authority to change the rules to suit the players, but do not be persuaded to do so often, or the rules will become less important.