Yes — you can move up to a higher site tier at any time, and what you already paid is credited toward the upgrade.
If you start with a single-site license and later need more sites, you can upgrade to a higher tier (for example a multi-site or unlimited license). You are not charged full price again: your current license price is deducted, so you pay only the difference, and your license term is extended to a year from the upgrade date.
You upgrade from your account at artifexrouting.com — for the exact steps, see Upgrade my license to more sites. Your existing license key keeps working after the upgrade; it simply covers more sites, which you then activate as usual (see Activate your license).
