A question:
Is the current OS install (that's on the internal HDD) doing well enough for you? Other than being slower than an SSD would be.
If so, you could do this:
a. you'll need either a USB3 enclosure or a USB3/SATA dongle/adapter. These will come in handy once you have the old drive "outside" -- to keep using it as a backup, extra storage, etc.
b. you'll need CarbonCopyCloner from here:
http://www.bombich.com/download.html
CCC is FREE to download and use for 30 days, so "doing it my way" won't cost anything (although I recommend buying the enclosure as stated above)
c. When you have the above items (and the new SSD), put it into the enclosure or connect to the dongle, then plug into the MacBook
d. You'll probably get a msg "this drive is unreadable" --
because there's nothing on it, so...
e. Open Disk Utility and erase it to Mac OS extended with journaling enabled (or perhaps APFS if you're using Mojave)
f. Once that's done, quit Disk Utility and open CCC
g. Accept all of CCC's default settings. Put your source (internal drive) on the left. Put the target (SSD) to the right. Then turn CCC loose.
h. CCC may ask if you wish to clone the recovery partition as well. YES, you want to do this.
i. When done, quit CCC and POWER OFF the MacBook.
j. Press the power on button and IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears.
k. Select the SSD with the pointer and hit return.
l. Do you get a good boot? If so, take a look around and make sure everything looks as it did on your HDD
m. OK, now it's time to power down and "do the drive swap".
n. Once you have the new SSD in, REPEAT the "option key bootup". When you get logged into the finder, go to startup disk pref pane and select the SSD as the boot drive.
o. DONE.
Do it this way, and I predict a success rate of better than 98%.
Print this msg out and save it for reference as you go along.
