By RAW, You would have to cast the spell again, but you have options.
There are a few relevant lines in Spell Combat:
This functions much like two-weapon fighting, but the off-hand weapon is a spell that is being cast. (d20PFSRD - Spell Combat Emphasis mine)
As a full-round action, he can make all of his attacks with his melee weapon at a –2 penalty and can also cast any spell from the magus spell list with a casting time of 1 standard action (any attack roll made as part of this spell also takes this penalty). (d20PFSRD - Touch Spells in Combat Emphasis mine)
This clearly states that the function of Spell Combat is to allow the casting of a spell to function as a second "weapon," not the spell itself.
All is not lost
Your first option is to just use the normal Two-Weapon Fighting rules on your next turn. This is because:
Alternatively, you may make a normal unarmed attack (or an attack with a natural weapon) while holding a charge. In this case, you aren’t considered armed and you provoke attacks of opportunity as normal for the attack. If your unarmed attack or natural weapon attack normally doesn’t provoke attacks of opportunity, neither does this attack. If the attack hits, you deal normal damage for your unarmed attack or natural weapon and the spell discharges. If the attack misses, you are still holding the charge. (d20PFSRD - Touch Spells in Combat Emphasis mine)
So if instead of using Spell Combat you simply use Two-Weapon Fighting and include an unarmed attack (and it hits) you get to discharge the spell you are holding.
Your next option is this feat, though it is third party and hence up to DM discression:
When holding a charge from a touch spell, you can choose not to have the spell take effect when you touch someone or something. You can cast other spells while holding a charge. If you cast another touch spell, you cannot hold the charge on the newly cast spell beyond the end of your turn, but you may deliver both that spell and the held touch spell in a single touch attack. You lose the held charge if you become unconscious or when you normally replenish your spells. (d20PFSRD - Touch Spell Control Emphasis mine)
This would allow you to just dive straight back in to Spell Combat.