'Custom and practice' has liitle, if any, relevance in contract law.
For example, when I worked on the railways, all staff got a free travel pass that could be used by any other person that they nominated. (Job adverts, published by the company, included that as one of the 'perks). However the company later switched to using the same system which most other train operators used, whereby such passes were only available to spouses, civil partners and people who were living with an employee as if they were their spouse or civil partner. That annoyed employees like me, who'd given their 'spare' pass to someone who was just a good friend, and who would no longer be eligible for one.
Four different trades unions got their solicitors involved and they were all told that (despite the perk being part of the advertised employment package) the free pass was only part of 'custom and practice', meaning that there was no legal right to retain the privilege.
I suspect that a lawyer would also say that your change in hours only formed part of 'custom and practice' and was therefore exempt from the provisions of contract law.