Reality checks:
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As your soldering skills aren't sufficient to remove a switch from the board, dealing with SMD soldering is likely well beyond what you should attempt.
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Current routers with greater capabilities are available for US$20 or less. Upgrading to 8M flash still leaves you with an underpowered, under-resourced router that will be obsolete and unable to handle current firmware reasonably soon (64 MB RAM on your W8970 will bite you very soon, 32 MB on the WR740N shown is already not enough for most applications).
tp-link used bad solder in the router
but i think can be cut pins of flash 8 MB and then solder flash 16 MB into old Pins !
i makes this pins but not easy remove the solder solder not melt easy
Flux - new solder into old not melt
but my solder is very easy to melt take 3 seconds with 60 watt soldering iron
the problem not in SMD soldering the problem is how to melt the solder into this router i Put soldering iron in this solder about 20 seconds and not melt ! bad solder ! i think more then 20 seconds damage track PCB !
but i think ram is enough now !
Use some new solder to melt the old one. An example how to desolder you will find here: https://www.youtube.com/watch?v=l6rw1zo4A2c&t=808s