In fact according to a recent report, it has been suggested that older Lumia phones with 8GB of storage of less will not be eligible for the update. This is according to a report from Windowsunited.de (via WMPoweruser) who claims to have spoken to Microsoft Support who in turn could not confirm if older entry-level devices would receive said update.
Surfing on over to Microsoft’s Windows 10 Mobile website sort of confirms that too. According to the website, it lists several devices that are eligible for the update, and adds that 8GB of internal storage is required. This means that even if your phone can be expanded via microSD, it does not matter if it does not have a base storage of 8GB.
It is unclear if Microsoft will find way to work around this limitation since we’re sure they would love to get Windows 10 Mobile on as many handsets as possible, but in the meantime we suggest you not get your hopes up just yet.