As long as you have an active monthly SMS plan (you get new
SMS-credits every month) you can not purchase another monthly SMS
credit plan. If you need more SMS-credits you can purchase regular credits.
If you have both monthly SMS-credits and regular credits. SMSMail will always automatically use the lowest cost SMS-credits available to you. So, if you have monthly credits these will always be used first and only if you have no monthly credits left will your regular credits be used. Regular credits never expire.
By "upgrade" we mean, switch from 50 SMS-credits per month to 100