Hello, and welcome back! Are you ready for a new challenge? In this unit, we're stepping up a notch to tackle a complex yet intriguing task. It involves parsing complex strings into Python dictionaries and then updating them, a common requirement in many real-world tasks. So yes, this unit's session is going to be pretty pragmatic — just the way you like it!

This task involves transforming a given string into a nested dictionary and updating a specific key-value pair within that dictionary. The input string will take the form "Key1=Value1,Key2=Value2,...". When a part of the value is another key-value string, we create a nested dictionary.

For example, the string "A1=B1,C1={D1=E1,F1=G1},I1=J1" should be transformed into the following nested dictionary:

Your Python function should parse this string into the above dictionary, then update the value of the nested key F1 from G1 to some other value, say 'NewValue'. The function should ultimately return the updated dictionary.

Our first step here will be to parse the string into a nested dictionary. Before diving into coding, let us understand that simple string splitting methods do not handle hierarchical structures well, hence, our solution needs to account for the nested nature of our text.

As you parse the string, you'll have to read through each character and note its role. Here's a hint: every time the '=' symbol is encountered, you can infer that the text bit you have on your hands is a key. Further, upon hitting the ',' symbol, you should gather that you've just finished reading a value. At this stage, it is safe to add the key-value pair you've just read into the dictionary you're constructing.