I have an existing game which I publish on the play store 2 years ago,the thing is I used different game engine back then and I want to make an update on the game and use C3 instead.Is that even legally OK?
that's totally fine, since the game is yours/game idea.... supposedly legally speaking .... you can prove that if someone sues you.
now ... if is not your game idea, and u just cloning it, u can do it, as long as u don't use the same assets (songs,images etc) and not limited to portions of the original code.
but since u will be using Construct and not normal coding( i mean the traditional one, construct coding is still normal coding, just filtered trough a visual editor), that's probably not going to be the case.
however.... if the idea of the game, was patented... and the original developer has a monopoly over the game(example like the tetris company and the game tetris )... then u are in a bit of a trouble, and i suggest just do ur own game and forget about the old one, but that doesn't mean u can't still clone it, and make it unique in its own way, gameplay might be similar but u have to have all interactions totally changed or be different then the original, in tetris game for example, the patent covers the shapes and the movement and even the background layout / game area pixel by pixel tile size by tile size and even how many tile counts are... so if u cloning tetris, the original one has 20 tiles as game area on vertical by 9 or 10 on horizontal, if u be doing, a 19 tiles vertical and a 12 tile horizontal won't be the same game anymore... however u have to have different shapes... and that is where many tetris cloners lost in court against law sues.
but if u are a publisher, and not the developer of the game u published... legally speaking, u can publish the original one again... but if u lost the source files... u have to go back to the developer and ask him to give u permission once more to use his code, or remake his game.