Of course they will change! And when they do, your financial planning may need to change with them. In an absolute worst-case scenario, you can stop paying in each month without losing the money you’ve invested (as long as you have completed the minimum “initial period”), and you can also withdraw from your savings when you need to. It’s not recommended to do this unless it’s for something big though – there are large loyalty bonuses for sticking with the plan as long as you can, and if you’ve done your financial planning properly you will have other places to take money from on a short term basis.
It’s far better to make sure at the start that you’re not over-committing yourself – pick a monthly amount that you can definitely afford, and integrate your savings planning with your overall financial planning – have an emergency fund to dip into for emergencies or unexpected expenses, and have some other ad-hoc savings and investments which you could withdraw from if needed, so you don’t need to compromise your long-term financial planning if you’re having some short-term financial difficulties.