My (extremely small) provincial company was sold (by me) 8 years ago. The larger company taking us over were required to keep staff contracts, and I got a new one as I was the only one without a contract of employment. If they wanted to change existing staff contracts this had to be by separate negotiation, and staff could, if they wish exercise all rights under employment law as if the new employer was the original one. Length of employment was seamless and continuous and cumulative.
Things may have changed but I suspect not.