Can mangas labeled as slash become shounen-ai, or do they have to be labeled shonen-ai from the start?

For shounen I doubt it. Shoujo or josei is where something like that is more likely to happen.

Woah. I never knew there was a 'slash' category before this.

In my opinion, I'd have to say no for both. It's sort of a subtext thing -- this thread explains it.

Also, I don't think manga that are labeled slash would become shounen-ai, especially with some shounen manga. Since certain shounen manga are serialized in a magazine targeted towards a male market, it'd be risky to turn it shounen-ai (i.e. Reborn! -- note: I have never read this manga before, but I'm familiar with the fact it's serialized in Shounen Jump and is pretty slashy).

For shoujo and josei it's sort of another thing...

I have never heard the term 'slash' used in the manga community, besides within fanfiction and original fiction which the use of the word is quite different.

I'm actual confused and actually not sure why non-BL manga are labeled slash to begin with. Slash is equivalent to the term BL/shounen-ai/yaoi so it's use as a category label seems false.

I would say yes.
Mangaka don't always have the plot set perfectly before they write; what begins as slash might become something more.
But once slash turns into shounen-ai, then it is no longer slash o.O

Quote from Chocoroll

Slash is equivalent to the term BL/shounen-ai/yaoi so it's use as a category label seems false.

I've always thought that slash can include straight pairings, too? Relationships that aren't canon, etc.