There is no way to reset master sso password without remembering old master sso password .See the KB VMware KB: Unlocking and resetting the vCenter Single Sign-On administrator password
You can try the following options .
1.) There are two password admin and master password. After installation of SSO 5.1 both passwords are same but if you change admin password through web client , admin password would be changed but Master password will still remain old one. It may help you to remember old password.
2.) You can uninstall sso 5.1 completely and re-install SSO 5.1 but you will lost all the data of the sso server like identity sources, users, sts etc. After re installing SSO 5.1 you need to re-register all the solutions as it is mentioned in previous comments.
3.) This is the best option for you , i feel so. You just upgrade only SSO 5.1 to SSO 5.5 , you don't need to provide old master password for this upgrade. Actually it is migrating the date from 5.1 sso to 5.5 SSO. It is kind of fresh installation of 5.5 but your data would be preserved. You don't need to re-register all the solutions , just restart all the services. It should work fine.
providing few links . VMware KB: Unlocking and resetting the vCenter Single Sign-On administrator password
Upgrade from sso.1 to 5.5 http://kb.vmware.com/selfservice/search.do?cmd=displayKC&docType=kc&docTypeID=DT_KB_1_1&externalId=2057258
FAQ SSO5.5 :VMware KB: VMware vCenter Single Sign-On Server 5.5 FAQs