Once you got back all your iPhone messages and media you MUST uninstall older WhatsApp and update to latest version from Google Play Store in order to get all features working

Before updating perform a LOCAL WhatsApp backup, as shown in the interactive tutorial below. Please note: Google Drive backup will be available once you update to latest version.

After LOCAL WhatsApp backup you can safely uninstall the old version and install the new one from Google Play Store. You'll be asked to activate WhatsApp again, this is expected and not an issue (you'll not loose the imported messages).