tconst Has Changed

I have noticed that the tconst for some movies has changed. I know this because I have the title basics file from different dates. For example. tt20913276 is now tt15944428. tt20913276 is no longer in the basics file and https://www.imdb.com/title/tt20913276/ now redirects to https://www.imdb.com/title/tt15944428/ Why has this happened. I thought tconst was constant and does not change.