Should I change vi-spring to a soft tension?

Hi John.
I found your website just after taking delivery of a VI Spring Traditional Bedstead in medium tension. I have a solid wooden slatted base. My old mattress, which was firm rated, proved to be too firm for me, I was waking with terrible aches and pains and particularly shoulder and neck pains causing headaches.
When looking to buy the VI Spring the only tensions available to try in store were medium and firm. I wasn’t asked what my weight was at the time, I am just under 10 stone and 5ft 6inches tall. I tend to sleep on my back and my side and like to read in bed on my side. Having had the mattress for three weeks now it is starting to soften up a little (it felt very firm first few nights) but I find that there still isn’t enough give for my shoulders to sink into the mattress when lying on my side, it feels comfortable enough when lying on my back. I am now concerned that I should have gone for the soft tension? I have bought a 3cm microfibre topper and this has given a slightly softer feel to the mattress but not enough to be able to sleep on my side.
As this has been a considerable long term investment for me I wonder if you could give me some advice. I do have the option of changing the tension to soft under their comfort pledge but am unsure about doing this because it is probably easier to soften up a medium mattress with a topper (deeper topper) than firm up a soft mattress if the soft proves too soft? I do not have a partner (and their weight to consider) at present but as this mattress should last many years that could change!
Looking forward to your reply.
Kim