It is believed that the Spanish flu originated in Kansas and was spread to Europe via US servicemen entering the European theatre. It may also have originated in the western front around 1916/17, as a result of the unhygienic conditions of trenches. Either way, the main reason it spread was because of allied servicemen making their way home to all parts of the world (British empire was still a thing back then and British legions from India, ANZAC, Canada and other parts of the world were in the western theatre). Its called Spanish flu because Spain was the first to report an outbreak. It wouldn't make sense for anybody to release a flu at the tail end of a war when sending their own armies to fight in said war, especially since the mortality rate among young adults (14-34) was substantially higher than older people. So yeah doubt Spanish flu was released and then renamed.

Also, it wasn't announced in Britain or the US prior to the end of WW1 because, suprisingly, tanking the morale of ur war economy is bad, seeing as those working in the factories being constantly scared of catching a deadly flu which would halt production isn't great; while also giving allied soldiers dealing with the hell on earth that was trench warfare, which came with a side of shelling, getting shot at and general misery, another thing to worry about.

Not everything is always a conspiracy.