Hi tempured mad,
The most common cause for shoulder issues or your arm falling asleep would either be a sleeping surface that is too firm or a pillow issue where your pillow isn’t keeping your head and neck in good alignment but it’s also possible that it’s a postural issue as well.
The first thing I would suggest is sleeping on your new topper for a few weeks so that it has a chance to break in a little and your body has a chance to adjust to a different sleeping surface.
Other than that … if your symptoms are from a pillow issue then you may need a thicker or thinner pillow (most likely a little thicker if your current sleeping surface is firmer and you are sinking in less than you were previously) to provide better support and alignment for your head and neck which can also affect the pressure on your shoulder.
It’s also possible that you are “going through” your topper and feeling too much of the firmness of the mattress underneath it (the memory foam in your mattress may not be softening as much as it used to when you were sleeping on it directly) and if this is the case you may need an additional topper to isolate you from the firmness of the mattress a little more but I would try the other options that are available to you before going in the direction of adding another topper that may or may not work for you (unless a topper you purchase has a good return/refund policy). Since the topper you have is fairly soft already if you do go in this direction I would probably suggest adding a medium topper in between the soft topper you have and the mattress rather than adding too much additional softness on top of your sleeping system which could result in alignment and lower back issues.
There are also some additional suggestions in posts #2 and #3 here that may also be helpful and would be worth trying before you spend any additional money on another topper that may not work as well as you hope it will.
Phoenix