Rulings
Q1: What does the "change 1 Digimon's original DP" effect do, exactly?
Such effects change the original DP shown on the target card to the predetermined value shown in the effect. For example, if a Digimon has an original DP of 8000 and it's chosen as the target for a "Main Change the original DP of 1 of your opponent's Digimon to 3000 until the end of their turn" effect, that Digimon's original DP will become 3000 instead of 8000.
Q2: What happens if a Digimon that got +1000 DP has its original DP changed by the "change 1 Digimon's original DP" effect?
Its original DP is changed, then +1000 is added to the changed DP. Values added to DP don't affect the original DP.
Q3: What happens if a Digimon has its original DP changed and is then affected by a "change 1 Digimon's original DP" effect again?
The original DP is changed to the most recent value.
Q4: Can a Digimon without DP be affected by the "change 1 Digimon's original DP" effect?
No, it can't. Effects that change information can't change information that a card didn't originally have. For example, if a Digimon has its original DP changed by an effect and then its top card becomes a card without DP due to a De-Digivolve effect, it will no longer be affected by the effect that changes the original DP.
Data Sources
Lv.5 Mistymon BT18-039 U Ultimate | Virus | Magic Warrior/Witchelny <Barrier> [On Play] [When Digivolving] By trashing your top security card, change 1 Digimon's original DP to 6000 until the end of of your opponent's turn. --- Inherited: [All Turns] [Once Per Turn] When your security stack is removed from, this Digimon may unsuspend.
Lv.5 Mistymon BT18-039 U Ultimate | Virus | Magic Warrior/Witchelny <Barrier> [On Play] [When Digivolving] By trashing your top security card, change 1 Digimon's original DP to 6000 until the end of of your opponent's turn. --- Inherited: [All Turns] [Once Per Turn] When your security stack is removed from, this Digimon may unsuspend.
Lv.5 Mistymon BT18-039 U Ultimate | Virus | Magic Warrior/Witchelny <Barrier> [On Play] [When Digivolving] By trashing the top card of your security stack, change the original DP of 1 Digimon to 6000 until the end of your opponent's turn. --- Inherited: [All Turns] [Once Per Turn] When a card is removed from your security stack, unsuspend this Digimon.
Lv.5 ミスティモン BT18-039 U 完全体 | ウィルス種 | 魔法戦士型/ウィッチェルニー ≪防壁≫ 【登場時】【進化時】自分のセキュリティを上から1枚破棄することで、相手のターン終了まで、デジモン1体の元々のDPを6000に変更する。 --- Inherited: 【お互いのターン】[ターンに1回]自分のセキュリティが減ったとき、このデジモンをアクティブにできる。