Yes, you can only apply for it if your GC expires in the next 6 months, but it takes around a year to get (see processing times), which means you will not get the new GC by the time the old GC expires. That's why you need an I-551 stamp for proof of permanent residence in the meantime before you get the new GC.Thank you for your response.
I'm not familiar with I-551 stamps, so I'll have to read up on them.
One thing I don't understand is that according to the I-90 form, I can only apply if my GC expires in the next six months. How can then it take a year to get it? I really thought (or hoped) it was a quicker process.
Thank you for all the information. We will apply for citizenship. I guess we'll see if they'll charge us for the I-551 stamps.