I cannot recreate your issue. I just deleted my Power 4.8.6 and Added Power 4.9 no problem, except that I forgot to delete the Power add-on and received an error message when i tried adding plugin, error message said file directory already existed. I do not know if the issue is because you are using PHP 5.3,
I will have to ask the developer. I am guessing that could be the issue, that version of PHP. I believe you would need at least 5.4 or higher.
PHP version 5.3 was in 2009…..is there any way you can update that?
The author/developer did not opt to auto update peoples SLP plugin and add-ons but he is considering doing so. I have mentioned that some people may have issues with that, such as the one you are experiencing. So it is something that he is considering and your feedback is helpful.
The problem with the add-ons not automatically being shown is because of the WP-CRON. When WP has a major update, like they just did, thousands and thousands of plugins also update, by the time it goes through all the thousands of plugins it may not get to the add-ons which are not sold in the WP directory. The SLP auhtor is aware of this and , again, that may be something he will do in the future (auto update when SLP updates)