Yes, most providers will allow you to change your funeral plan, though it's worth keeping a few things in mind:
Some providers only allow you to make changes to your plan once you've paid for it in full
You may only be able to make a set number of changes to your plan, for example, only changing it once a year
If you add extra items or requests to your plan you'll need to pay extra to cover the additional costs
Each funeral plan provider is slightly different, so it's always worth checking with your provider about what changes you can make.