Can Changing your Permalink Structure Alter Previous Links from External Sources

I have probably created a monster, previously my permalinks were very long winded and messy (so I thought)

I have played with the settings (yes I really am that thick) and have altered to website and post title only. However someone has told me that all my previous links are going to be effected …

I am not the most technical (as you can tell from the fact i played first and asked questions later) however I would love some info/help on how this is going to effect my site re links/google and also if it needs fixing how do i go about that?

Thank you in advance